In the history of biological thought, few investigations represent so clean an epistemological rupture with reigning orthodoxy as the experiments carried out in the summer of 1849 by the German zoologist and physician Arnold Adolph Berthold. Working in the provincial academic setting of the University of Göttingen, Berthold performed a deceptively simple series of surgical manipulations on six young domestic cockerels (Gallus gallus domesticus). By castrating these animals and subsequently translocating their testes into ectopic sites within the peritoneal cavity, he detached the organs completely from their native anatomical nervous connections. In doing so, he observed a phenomenon that the prevailing physiological paradigms of the nineteenth century were wholly unequipped to explain: the transplanted gonads, despite having no connection to the autonomic or central nervous systems, maintained the somatic morphology, vocalizations, sexual appetites, and agonistic behaviors characteristic of fully intact adult roosters.
This empirical demonstration dealt an unanticipated blow to the neurocentric doctrine of bodily coordination that had dominated European physiology since the decline of classical humoralism. At a time when the nervous system was regarded as the sole and sovereign coordinator of distant organ systems, Berthold’s deductive leap—that the testes exert their profound systemic influences not via nerve pathways, but through an alterative property communicated directly to the circulating blood—inaugurated the conceptual framework of internal secretion. His short four-page treatise, published in Johannes Müller’s prestigious anatomical journal, received scant contemporary notice and languished in relative obscurity for nearly half a century. Yet, when viewed retrospectively through the prism of early twentieth-century biochemistry, Berthold’s elegant design stands as the foundational cornerstone of modern experimental endocrinology.
To fully grasp the magnitude of Berthold’s achievement requires more than a cursory review of his surgical protocol. It demands an exhaustive exploration of the historical, theoretical, and technical milieu of mid-nineteenth-century medicine. One must appreciate the profound grip of reflex theory on the minds of early physiologists, the centuries-old agrarian empiricism of caponization, the daunting technical challenges of pre-antiseptic and pre-anesthetic avian surgery, and the tortuous intellectual trajectory that ultimately led from Berthold’s ectopic gonadal grafts to the chemical isolation of testosterone and the mapping of the hypothalamic-pituitary-gonadal axis. Berthold’s rooster testicle transplant experiment stands not merely as an isolated biological curiosity, but as a methodological masterpiece that redefined the very nature of organismal communication.
1. Introduction to Arnold Adolph Berthold and the Pre-Endocrine Era
1.1 Biographical Context of Arnold Adolph Berthold
Arnold Adolph Berthold was born on February 26, 1803, in the Westphalian town of Soest, then part of the Prussian state. Reared in a cultural climate deeply influenced by the late German Enlightenment and the burgeoning empirical natural sciences, Berthold demonstrated an early affinity for medicine and comparative zoology. He pursued his university studies at the renowned University of Göttingen, an institution distinguished throughout the eighteenth and nineteenth centuries as an epicenter of European scientific rigor and experimental naturalism. Under the mentorship of eminent scholars—including the comparative anatomist and physical anthropologist Johann Friedrich Blumenbach, as well as the surgeon and anatomist Konrad Johann Martin Langenbeck—Berthold acquired a profound mastery of anatomical dissection, comparative organology, and clinical surgery, graduating with his medical doctorate in 1824.
Following his graduation, Berthold undertook scholarly journeys to medical and zoological institutions across Berlin, Paris, and London, exposing him directly to European developments in experimental physiology. Returning to Göttingen, he established himself as a Privatdozent in 1825, rapidly ascending the academic hierarchy. By 1836, he was appointed an extraordinary professor, and in 1840, he achieved the status of full professor of medicine and physiology, simultaneously serving as curator of the university’s expansive zoological collections. Berthold was a polymath whose academic curiosities transcended the boundaries of human medicine; his published corpus embraced comprehensive treatises on herpetology, avian anatomy, entomology, human dermatological physiology, and teratology. His text Lehrbuch der Physiologie der Menschen und der Thiere (1837) highlighted his firm conviction that human somatic functions could only be elucidated by comparative investigations across the animal kingdom.
The intellectual climate of Göttingen during Berthold’s tenure was characterized by a distinct balance between the waning romanticism of German Naturphilosophie and the emergent ethos of inductive, mechanistic laboratory science. Rather than viewing living systems through mystical teleologies or pure speculative metaphysics, the Göttingen school insisted upon rigorous observation, surgical reproducibility, and anatomical dissection. It was precisely this convergence of comparative zoological curiosity, surgical dexterity, and empirical discipline that positioned Berthold to interrogate the systemic effects of gonadal manipulation with a level of experimental clarity that had eluded all of his clinical and agricultural predecessors.
1.2 State of Physiological Knowledge in 1849
To understand the revolutionary character of Berthold’s work, one must contextualize the physiological universe of 1849. The early nineteenth century was marked by an intellectual struggle between mechanistic vitalism and reductionist biophysics. The pioneering cell theories advanced by Matthias Jacob Schleiden in 1838 and Theodor Schwann in 1839 were only beginning to reshape histology, establishing that tissues were composed of individual, self-propagating cellular units. However, the mechanisms through which these disparate cellular aggregates coordinated their metabolic, developmental, and functional activities across the expanse of a complex metazoan body remained an enigma. The prevailing view recognized only two principal mechanisms of systemic physiological cohesion: mechanical forces (such as hemodynamics and hydrostatic pressure) and the electrical or energetic impulses transmitted through the nervous system.
During this period, the concept of a hormone, an endocrine gland, or a “chemical messenger” was completely non-existent. While the ancient humoral doctrine formulated by Hippocrates and Galen had postulated that health and disease were dictated by the balance of four vital humors (blood, phlegm, black bile, and yellow bile), the scientific revolution of the seventeenth and eighteenth centuries had largely discredited humoralism as superstitious and unscientific. In its place, the ascendant disciplines of gross anatomy and neurophysiology elevated the nervous system as the sole coordinator of bodily balance. Tissues and organs were conceived as passive anatomical endpoints operated by an intricate web of neural cables, reflexes, and sympathetic ganglia.
Paradoxically, while academic physiology lacked any concept of internal secretions, European agrarian society possessed an exhaustive, empirical familiarity with the phenotypic consequences of gonadal removal. For thousands of years, farmers, livestock breeders, and bird fanciers had practiced the surgical castration of young male domestic fowl to produce capons—birds prized for their docile temperament, accelerated fattening, and tender culinary qualities. It was widely known that removing the testes of a young cockerel stunted the growth of its comb and wattles, extinguished its aggressive territorial drives, abolished its early morning crowing, and eliminated its reproductive libido. Yet this agrarian knowledge remained entirely disconnected from theoretical biology. No contemporary scientist had established how an isolated pair of small ovoid organs situated deep within the sub-lumbar avian coelom could exert such vast somatic and behavioral dominance over tissues located at the opposite extremity of the animal’s body.
1.3 Historical Significance of the Göttingen Experiments
The definitive breakthrough occurred when Berthold designed an experimental protocol that transitioned the study of gonadal action from passive observation to active, controlled surgical intervention. In 1849, Berthold published his landmark paper, titled “Transplantation der Hoden” (“Transplantation of the Testes”), in the prestigious journal Archiv für Anatomie, Physiologie und wissenschaftliche Medicin, edited by the preeminent German physiologist Johannes Müller. Spanning a mere four pages, this modest communication documented an experiment so theoretically pure, parsimonious, and analytically coherent that it would subsequently be heralded as the foundational event of experimental endocrinology.
Berthold did not merely perform castrations; he executed autologous and allogeneic testicular reimplantations into anatomical compartments entirely separated from the original gonadal site, explicitly severing all nervous connections and surgical tethers. By proving that a testis grafted freely into the peritoneal cavity onto the serosal surface of the intestines was capable of preserving masculine morphology, plumage, vocal displays, and instinctual behaviors, Berthold directly demolished the assumption that secondary sexual characteristics were sustained through sympathetic or central innervation. The experiment represented a watershed epistemological leap: it demonstrated that an organ could dictate systemic physiology through a vehicle other than the nervous reflex arc.
Retrospectively, Berthold’s 1849 experiment is revered as the first rigorous application of the classical endocrine paradigm: ablation, observation of deficit, replacement, and phenotypic rescue. By demonstrating that the vascular system carries an active biological signal generated by an organ to distant somatic targets, Berthold laid the conceptual foundation upon which the entire modern discipline of endocrinology was erected. He transformed what had been an empirical craft practiced by rural poultrymen into a precise biological science, shifting Western medicine from the remnants of humoral mysticism and the confines of rigid neural determinism into the era of vascular-borne chemical signaling.
2. Nineteenth-Century Physiological Paradigms: The Dominance of Neural Theory
2.1 The Neurocentric Model of Bodily Coordination
To appreciate the theoretical resistance Berthold’s findings encountered, one must examine the absolute dominance of the neurocentric model of physiology during the first half of the nineteenth century. Driven by the groundbreaking discoveries of Luigi Galvani, Alessandro Volta, and later Emil du Bois-Reymond, the scientific establishment viewed electrical and nervous impulses as the ultimate animating forces of biological organisms. The nervous system was conceptualized as a vast network of physical telegraphy, an intricate biological infrastructure that linked disparate sensory receptors to executive centers in the spinal cord and brain, which in turn dispatched regulatory motor and vasomotor instructions to periphery organs.
This mechanistic paradigm received immense empirical reinforcement through the formulation of the reflex arc by the English physician Marshall Hall and the systematic elaboration of neural reflex actions by Johannes Müller. Müller’s doctrine of “specific nerve energies” posited that sensory nerves produce distinct modalities of sensation depending on the specific central termination of the nerve fibers, reinforcing the concept that all physiological perception, somatic coordination, and homeostatic regulation were strictly dependent on anatomical neural continuity. Under this framework, if an organ altered its functional output in response to events occurring elsewhere in the organism, the communication was assumed to have occurred via an afferent neural signal entering a sympathetic ganglion or the spinal cord, followed by an efferent neural discharge back to the responsive tissue.
Consequently, the scientific community operated under the dogmatic assumption that the systemic influences exerted by the gonads were purely neurogenic. The profound somatic changes following castration—such as the collapse of cranial ornaments or changes in temperamental aggression—were theorized to result from the abrupt termination of afferent neural signals originating in the rich sympathetic nerve plexuses that encase the reproductive organs. Physiologists hypothesized that the testes maintained a perpetual tonic reflex via the celiac, pelvic, and mesenteric nerves, continuously stimulating the central nervous system to direct head vascularization, vocal-cord development, and aggressive territorial drives. A non-neural, circulating communication vector was viewed with deep skepticism, dismissively associated with the obsolete metaphysical archetypes of ancient medicine.
2.2 Traditional Agrarian Practices of Caponization
While nineteenth-century academic physiology remained constrained by neural dogmas, the somatic and behavioral sequelae of testicular removal had been thoroughly cataloged by agricultural societies for millennia. The practice of caponization—the surgical ablation of the testes in juvenile male domestic poultry—dates back to classical antiquity, with detailed descriptions recorded in the agrarian manuals of Cato the Elder, Columella, and the natural histories of Aristotle. The primary agrarian motive was purely economic: uncastrated male roosters channel substantial metabolic energy into constant territorial patrolling, dominance combat, persistent courtship, and loud vocalizations, resulting in lean, fibrous, and tough meat. Castration transformed the combative cockerel into a placid, docile animal that ceased wasting energy on sexual and aggressive pursuits, converting dietary calories into subcutaneous adipose tissue and delicate, tender muscle mass.
The morphological alterations triggered by caponization were striking and unvarying across all avian breeds. Within weeks of successful testicular excision, the young bird’s prominent, bright-red, fleshy comb and pendulous wattles—the primary vascular secondary sexual ornaments of the male cock—ceased growing, lost their vivid scarlet hue, turned pale and flaccid, and underwent severe atrophy. The somatic transformation extended to plumage and skeletal development: while feather shape and length were influenced by complex genetic and metabolic determinants, the general lustrous sheen, aggressive stance, and vocal cadence of the male bird completely degraded. Capons no longer greeted the dawn with proud crowing; their vocalizations were reduced to weak, juvenile chirps and clicks.
Behaviorally, the capon underwent a complete personality collapse. The innate territorial instinct that compels an intact cockerel to defend his territory against rival males and aggressively court receptive hens vanished. Capons exhibited complete sexual apathy, living alongside both hens and roosters without provoking or engaging in agonistic encounters. Yet, despite thousands of years of performing this surgery, agrarian practitioners viewed it merely as an empirical technique. Naturalists and early veterinarians failed to investigate the biological bridge connecting the physical presence of the testes with these diverse, remote somatic and behavioral phenomena. The capon was seen simply as a manufactured creature, an agricultural commodity whose bizarre transformations were taken for granted rather than recognized as a critical physiological problem.
2.3 Early Speculations on Humors versus Nerves
Beneath the surface of nineteenth-century mechanistic physiology, a quiet intellectual conflict simmered between the fading relics of humoralism and the rising architecture of neurocentrism. While the ancient Galenic system of four cardinal humors had been dismantled by the rise of Morgagnian morbid anatomy and Virchow’s burgeoning cellular pathology, a handful of visionary thinkers wondered whether the blood might possess complex, non-mechanical regulatory properties. French physician Théophile de Bordeu had posited in the mid-eighteenth century that every organ of the body emits a specific “emanative tone” or dynamic exhalation into the vascular system, which preserves the overall harmony of the organism. However, Bordeu’s musings were speculative, devoid of experimental proof, and tinged with vitalistic mysticism, causing mainstream physical-chemical physiologists to dismiss them.
The central epistemological obstacle preventing early nineteenth-century researchers from validating a blood-borne mechanism of organ action lay in the immense technical difficulty of separating vascular perfusion from tissue innervation. In living animals, blood vessels and nerve trunks travel together in dense neurovascular bundles. Whenever an investigator surgically ablated an organ, severed its vascular pedicle, or ligated its vascular supply, they inevitably severed the autonomic nerve fibers running alongside the arterial walls. Conversely, severing the nerve plexuses often compromised local vasomotor tone, triggering secondary ischemia and tissue necrosis.
To definitively prove that an organ exerted its systemic effects via the blood rather than through the nervous system, an investigator needed to devise an experiment capable of physically decoupling tissue innervation from vascular survival. One had to place the target organ in an anatomical environment where it could receive nutrients and discharge substances into the bloodstream while remaining wholly isolated from any native, sympathetic, or reflex-generating neural connections. In the mid-nineteenth century, such an experimental design seemed surgically unattainable and theoretically implausible. The intellectual inertia of the era dictated that if a biological effect occurred, a nerve was responsible; to think otherwise required a radical departure from the academic consensus of 1849.
3. The Experimental Catalyst: Formulating the Capon Hypothesis
3.1 The Core Scientific Question
Arnold Adolph Berthold approached this deep-seated physiological problem with an uncommon clarity of purpose. Rather than accepting the vague assertion that the testes were simply “organs of generation” whose loss provoked a non-specific somatic depression, Berthold framed a precise, falsifiable research question: Do the testes govern male secondary sexual characteristics and behavioral instincts via an intact connection to the nervous system, or does their biological influence operate through the bloodstream? This question strikes at the core of systemic biology: it challenged the foundational assumption of his era by asking whether an organ requires anatomical integration into the neural network to direct distant somatic and neurological functions.
To answer this question, Berthold recognized that he had to construct a biological scenario in which the putative neural conduits of gonadal communication were permanently eliminated, while the functional viability of the testicular parenchyma was rigorously maintained. His working hypothesis was audacious: if the profound somatic changes observed in caponization—the atrophy of the comb, the loss of crowing, the cessation of male combat, and the disappearance of sexual libido—resulted exclusively from the interruption of neural reflex arcs, then removing a testicle from its natural sub-lumbar site and re-implanting it in an ectopic location devoid of native innervation should produce a phenotypic capon. If, however, the ectopic testicular tissue successfully retained its ability to preserve the masculine phenotype and behavioral repertoire, one would be logically compelled to conclude that the testes act upon the organism via the vascular system, operating independently of the central and peripheral nervous networks.
Furthermore, Berthold sought to determine whether the physiological effects of gonadal presence were fundamentally reversible. While agrarian practice demonstrated that the removal of gonads halted the progression of male traits, it had not established whether an established capon could be functionally rescued through late surgical reimplantation, or whether an ectopic testicular graft could prevent the onset of caponization altogether. By formulating his hypothesis around the definitive dissociation of nerve from vessel, Berthold designed an experiment that directly tested the absolute limits of the neurocentric model.
3.2 Selection of Gallus gallus domesticus as an Animal Model
The selection of the experimental animal model was crucial to the success of Berthold’s study. His background in comparative zoology led him to select Gallus gallus domesticus, the domestic cockerel. The domestic fowl offered exceptional biological advantages that made it infinitely superior to mammalian subjects for investigating secondary sexual characteristics. Mammalian secondary sexual dimorphism is often subtle, slow to develop, or anatomically obscured within internal structures or diffuse behavioral nuances. In contrast, the male domestic rooster exhibits an exaggerated, unambiguous, and rapidly responsive suite of sexually dimorphic traits.
Foremost among these morphological markers are the cranial appendages: the fleshy, highly vascular comb (crista carnea) and the paired pendulous wattles (palea). These ornaments are exquisitely sensitive to the biological influence of the testes; their size, turgidity, and deep crimson pigmentation serve as immediate, visible indicators of the bird’s internal androgenic status. In addition to these cranial furnishings, roosters possess striking dimorphism in their plumage—exhibiting elongated, pointed neck hackles, decorative back feathers, and prominent curving tail sickles—alongside specialized skeletal structures such as calcified tarsometatarsal spurs. Ethologically, the rooster exhibits an unmistakable repertoire of quantifiable, stereotyped behaviors: aggressive territorial vocalizations (crowing), rigid peck-order combat with conspecific males, elaborate courtship waltzes before hens, and forceful copulatory mounting displays.
Beyond these phenotypic assets, the domestic fowl possessed crucial anatomical and logistical characteristics that facilitated Berthold’s surgical goals. Cockerels were readily available, economically manageable, and robust enough to withstand significant physical trauma. Most importantly, the avian coelomic cavity—unlike the mammalian peritoneal space—features extensive air sac networks and anatomical configurations that permit relatively rapid access to the retroperitoneal sub-lumbar space. Avian species also exhibit an extraordinary capacity for localized tissue repair, microvascular adaptation, and post-operative survival, rendering them uniquely suited to survive invasive intra-abdominal surgery in an era long preceding the development of modern veterinary intensive care.
3.3 Anticipated Methodological Challenges
While the avian model offered undeniable biological advantages, the technical execution of intra-abdominal surgery in 1849 was fraught with perilous obstacles. Modern surgical standards were completely non-existent: Joseph Lister would not introduce carbolic acid antisepsis until 1865, and the germ theory of disease championed by Louis Pasteur and Robert Koch was still decades away. Any surgical incision through the lateral body wall and into the coelom carried an immense risk of introducing environmental contaminants, almost invariably resulting in acute septic peritonitis, generalized systemic sepsis, and fatal post-operative shock.
Compounding the problem of infection was the total absence of modern inhalational or systemic chemical anesthesia. The anesthetic properties of diethyl ether had been demonstrated by William T. G. Morton in Boston only three years prior, in October 1846, and chloroform had been introduced by James Young Simpson in late 1847. These revolutionary agents, however, were not yet integrated into standard animal laboratory operations in continental Europe, nor were their pharmacokinetics, respiratory depression profiles, and margin of safety understood for avian physiology. Berthold was forced to rely entirely upon physical restraint, requiring swift, confident surgical execution to minimize the physiological trauma, autonomic distress, and hemodynamic shock endured by the conscious birds.
Finally, the surgical removal of the avian testes presented immense intraoperative risks. The avian gonads do not descend into an external scrotal sac as they do in most placental mammals; they remain situated deep within the dorsal coelom, anchored sub-lumbarly against the anterior poles of the trilobed metanephric kidneys, directly overlying the inferior vena cava and the descending dorsal aorta. The testicular capsules are exceptionally friable and receive their arterial perfusion through short, wide spermatic arteries that branch directly off the major abdominal vasculature. A fractional millimeter deviation of the surgical scalpel, or an uncontrolled tear of the short vascular pedicle during testicular avulsion, would lead to catastrophic, uncontainable internal hemorrhage, killing the subject in seconds. Furthermore, Berthold faced the daunting biological uncertainty of whether an excised testis, stripped of its primary vascular pedicle and tossed loosely into the peritoneal cavity, could ever establish sufficient spontaneous microvascular perfusion to avoid acute ischemic necrosis, liquefaction, and toxic reabsorption.
4. Experimental Architecture: Groups, Cohorts, and Controls
4.1 Cohort Division and Sample Size Configuration
To execute his investigation with maximum scientific rigor, Berthold established an experimental architecture that anticipated modern principles of controlled biological trial design. Eschewing the haphazard, single-subject observations that characterized many early nineteenth-century medical reports, he assembled a standardized cohort of six young male cockerels (numbered 1 through 6). These birds were of identical developmental age—approaching early adolescence, prior to the full, adult manifestation of their sexual characteristics—and hailed from the same domestic genetic stock, thereby minimizing confounding variables associated with developmental maturity, seasonal hormonal fluctuations, and genetic phenotypic variation.
Berthold divided these six birds into three distinct, carefully coordinated experimental cohorts of two subjects each. This paired configuration (n=2 per condition) provided an indispensable internal verification mechanism; it ensured that the physiological and behavioral consequences observed in any individual cockerel were not the spurious artifacts of idiosyncratic surgical trauma, atypical anatomical variation, or subclinical disease. While a sample size of six subjects would be considered modest by contemporary high-throughput statistical paradigms, it represented an exemplary, highly disciplined implementation of the comparative method for nineteenth-century descriptive and functional biology. The structural configuration of the cohorts was designed as follows:
- Cohort 1 (Birds 1 and 2): Bilateral orchidectomy without tissue reimplantation (The Negative Control / Classic Capon Group).
- Cohort 2 (Birds 3 and 4): Bilateral orchidectomy followed by immediate, autologous reimplantation of one excised testis into the subject’s own peritoneal cavity.
- Cohort 3 (Birds 5 and 6): Bilateral orchidectomy followed by reciprocal, allogeneic cross-transplantation of a single excised testis between the two subjects into their respective peritoneal spaces.
Through this architectural design, Berthold constructed a multi-layered matrix of negative and positive baseline comparisons, systematically isolating the presence of the gonadal tissue, its anatomical position, its neural continuity, and its genetic-individual origin as independent experimental variables.
4.2 Group 1: Bilateral Orchidectomy (The Capon Control)
Cohort 1, comprising cockerels 1 and 2, was designated as the definitive negative control group. In both of these juvenile birds, Berthold performed complete bilateral surgical excision of the testes, deliberately discarding the extracted organs and leaving the abdominal coelom entirely devoid of gonadal tissue. The primary scientific objective of this group was twofold: first, to definitively reproduce the classical morphological and behavioral phenotype of the agrarian capon under laboratory conditions, and second, to verify that the physical trauma, surgical incisions, intercostal tissue disruption, and blood loss associated with the operative procedure were not, in and of themselves, the underlying cause of any subsequent somatic or behavioral regression.
By establishing this negative baseline within the exact same experimental run, using identical housing, nutritional regimens, environmental temperatures, and surgical timelines as the experimental subjects, Cohort 1 provided an essential reference point. If the surgical trauma alone induced a permanent suppression of secondary sexual characteristics, then all subsequent cohorts would be uninterpretable. If, however, Cohort 1 reliably exhibited the complete cessation of comb growth, loss of vocalization, and social pacification typical of capons, while cohorts receiving transplanted tissue diverged from this somatic trajectory, Berthold could confidently attribute the phenotypic divergence directly to the presence and metabolic activity of the translocated gonadal mass.
4.3 Group 2: Castration with Autotransplantation
Cohort 2, consisting of cockerels 3 and 4, represented the first experimental intervention: castration combined with autologous tissue transplantation. In these animals, Berthold performed the same bilateral orchidectomy, but with a critical operational variation. Rather than discarding both extracted organs, he carefully conserved one of the excised testes and immediately reimplanted it deep into the bird’s own abdominal coelom. The testis was deposited among the tortuous loops of the small intestine, deliberately distant from the original sub-lumbar retroperitoneal bed, and with absolute disregard for its original orientation or tissue beds.
The profound theoretical utility of Cohort 2 lay in its clean decoupling of organ viability from neural architecture, while simultaneously eliminating the confounding variable of immunological tissue rejection. Because the grafted testis was harvested from and returned to the exact same host organism (an autograft), the subject’s host tissue would not mount an allo-immune response against the transplanted parenchyma. This cohort served to answer a foundational question: Could an isolated testicle, severed from its primary spermatic nerve supply, its native arterial pedicle, its venous drainage, and its excretory ductus deferens, survive in an entirely alien anatomical landscape and sustain the normal masculine development of the host? If Birds 3 and 4 developed into fully virilized roosters despite their gonads resting loosely upon their bowels, the doctrine of direct nervous stimulation would be irrefutably undermined.
4.4 Group 3: Castration with Allotransplantation
Cohort 3, consisting of cockerels 5 and 6, expanded the conceptual boundary of the experiment even further by introducing the variable of allogeneic transplantation—the cross-grafting of tissue between two genetically distinct individuals of the same species. In this cohort, both birds underwent complete bilateral castration. However, instead of receiving their own testicular tissue back into their peritoneal cavities, Berthold performed a reciprocal transfer: the excised testis from Bird 5 was surgically deposited into the peritoneal cavity of Bird 6, and the excised testis from Bird 6 was concurrently implanted into the coelom of Bird 5.
This design represented an exceptional intellectual gambit for 1849. Berthold sought to interrogate the species-wide autonomy of the testicular signal. He aimed to ascertain whether the putative physiological influence of the testis was an idiosyncratic, strictly individualized phenomenon tied to the unique nervous or somatic constitution of the original host, or whether it represented a universal, biologically conserved signal operating seamlessly across different organisms of the species. Furthermore, by executing reciprocal cross-transplantations, Cohort 3 evaluated whether non-autologous gonadal tissue could successfully engraft, survive, and vascularize within the peritoneal environment of an unrelated host. Cohort 3 stood as the definitive test of whether gonadal signaling was mediated by a systemic, blood-borne medium that functioned universally, independent of individual nervous wiring or unique bodily identity.
5. Surgical Protocols and Operative Techniques in 1849
5.1 Pre-Operative Preparation and Restraint
Executing abdominal surgery on conscious avian subjects in the mid-nineteenth century required extraordinary manual dexterity, profound anatomical familiarity, and exceptional physical restraint. In the pre-anesthetic era, pre-operative preparation began with the rigid physical immobilization of the young cockerels. Berthold, assisted by laboratory aides, secured the birds upon their sides upon a wooden operating surface. The wings were folded securely back and tied or manually restrained to prevent sudden flapping, while the pelvic limbs were extended posteriorly and tied downward to expose the lateral body wall and hypochondriac regions.
To establish a clear, unobstructed surgical field, the feathers overlying the lateral margins of the abdomen, the flank, and the area between the last two ribs were meticulously hand-plucked down to the bare epidermal surface. The skin was cleansed using basic water, mechanical wiping, and rudimentary sponges—interventions aimed solely at removing gross organic debris and loose feathers rather than achieving chemical antisepsis, which remained undiscovered. Berthold had to maintain a precise mental map of the underlying visceral topography. He was keenly aware that domestic fowl possess a delicate anatomical framework characterized by fragile, highly vascular pneumatic bones, a non-expandable avian lung structure closely apposed to the thoracic cage, and a complex system of thin-walled abdominal and thoracic air sacs that, if torn indiscriminately or flooded with blood, could lead to rapid asphyxiation or pneumothorax.
5.2 The Surgical Castration Technique
The surgical approach adopted by Berthold mirrored the traditional avian caponization techniques refined by experienced rural surgeons, albeit executed with the precise anatomical tools of a university prosector. Making a firm, controlled incision through the cutaneous layer and the thin external and internal oblique abdominal muscles between the last two ribs, Berthold entered the lateral aspect of the avian coelomic cavity. This incision breached the transparent wall of the lateral abdominal air sac, granting direct visual and physical access to the retroperitoneal viscera. The operating field was narrow, poorly illuminated by natural daylight or oil lamps, and continuously obscured by the rhythmic respiratory movements of the subject.
Berthold gently deflected the loops of the jejunum and ileum ventrally and medially using blunt retractor hooks, revealing the upper dorsal ceiling of the coelom. There, nestled immediately anterior to the cephalic lobes of the metanephric kidneys, lay the paired avian testes—glistening, pale-yellow, oval bodies. The primary operational crisis in avian castration is the extreme vulnerability of the regional vascular architecture. The testes reside in intimate, direct contact with the vena cava and the descending dorsal aorta; the short, wide testicular veins empty directly into the vena cava with virtually no intervening pedicle. Berthold utilized delicate surgical forceps, blunt hooks, and a slender, specialized scoop or miniature scalpel to carefully free the testicular capsule from its peritoneal reflections. With a deliberate, twisting avulsion maneuver designed to encourage primary vascular spasm and thrombosis, he extracted the gonads, taking painstaking care to ensure that the entire organ was mobilized intact. Any microscopic remnant of the testicular parenchyma inadvertently left adhering to the vena cava or lumbar fascia could rapidly hypertrophy and regenerate, invalidating the surgical ablation.
5.3 The Implantation Procedure
Once the bilateral excision of the testes was complete, Berthold immediately transitioned to the transplantation phase for Cohorts 2 and 3. In Cohort 2, one of the harvested testes from the bird was immediately reintroduced through the lateral abdominal incision. For Cohort 3, the harvested testis was transferred across the operating table and placed into the coelomic cavity of the paired recipient. The organ was deposited loosely into the lower peritoneal cavity, positioned deliberately among the complex, mobile loops of the small intestine and their associated mesenteries.
Crucially, Berthold made no attempt—nor did nineteenth-century surgical technology permit him—to perform microvascular anastomosis or neural coaptation. The excised testicle was placed entirely free within the peritoneal space, completely divorced from its original anatomical position, its severed vas deferens, its vascular pedicle, and its rich plexus of mesenteric and aortic autonomic nerves. The organ lay as an unanchored, free-floating tissue graft, fully reliant on the passive bathing of coelomic peritoneal fluid and whatever subsequent inflammatory adhesions and spontaneous microvascular sprouting might occur between its tunica albuginea and the highly vascular serosa of the host’s intestinal loops. Following the placement of the graft, Berthold closed the muscular and cutaneous layers of the flank incision using simple interrupted silk sutures, cleansed the wound site of blood, and released the birds into their convalescent recovery quarters. The birds were monitored intensely for signs of peritonitis, acute internal hemorrhage, and post-operative shock, with Berthold recording their systemic recovery and behavioral vitality.
6. Morphological and Somatic Observations Across Test Cohorts
6.1 Phenotypic Trajectory of Group 1 (Castrates)
Following their surgical convalescence, the cockerels in Cohort 1 (the bilaterally castrated negative controls) exhibited a dramatic and predictable somatic divergence from their intact peers. Over the ensuing weeks and months of observation, Birds 1 and 2 underwent complete phenotypic caponization, providing a striking baseline of unmasculinized male avian development. The most immediate and quantifiable morphological regression occurred in their cranial ornaments. The comb and paired wattles ceased their developmental hypertrophy; instead of burgeoning into large, turgid, highly vascularized adult structures, they underwent profound atrophy. The comb shriveled, collapsed against the dorsal surface of the cranium, and lost its vibrant, oxygenated scarlet coloration, fading to a pale, yellowish-gray, desiccated appearance.
The somatic alterations extended well beyond the cranial appendages. The castrated birds exhibited marked changes in their plumage patterns, body composition, and physical posture. While capons continue to grow and indeed often achieve a total body mass exceeding that of intact roosters due to the unimpeded accumulation of subcutaneous and visceral adipose tissue, they lack the dense, firm muscularity and high metabolic posture of virile cocks. Birds 1 and 2 moved with a sluggish, unassertive gait, failing to exhibit the upright, vigilant, chest-forward stance characteristic of territorial roosters. Their plumage, though long, lacked the radiant gloss and structural tension induced by normal male hormonal cycles. The head and neck furnishings appeared diminutive and weak, cementing their transition into somatic neutrals—animals structurally capable of growth, but completely devoid of the masculine morphological architecture.
6.2 Phenotypic Preservation in Group 2 (Autografts)
In striking and unambiguous contrast to the castrated controls of Cohort 1, the cockerels in Cohort 2 (Birds 3 and 4), which had undergone bilateral castration followed by autologous peritoneal reimplantation, completely maintained their male somatic developmental trajectory. Rather than undergoing the profound atrophy observed in the capons, the cranial furnishings of Birds 3 and 4 flourished. The combs and wattles of these birds grew vigorously, achieving the full dimensions, structural turgor, and vivid, engorged crimson pigmentation characteristic of robust, non-operated adult domestic roosters.
The physical transformation of Birds 3 and 4 into fully realized masculine archetypes was flawless. Their plumage displayed the complete morphological suite of male adornment: long, sharply pointed neck hackles, glistening saddle feathers cascading over their rumps, and dramatically arched, lustrous tail sickles that caught the light with deep green and bronze iridescence. Their leg spurs lengthened and hardened into formidable calcified weapons. Somatically, histologically, and morphologically, Birds 3 and 4 were utterly indistinguishable from intact, normal cockerels that had never undergone surgical intervention. The fact that their testes no longer resided in the sub-lumbar region, had zero connection to the spermatic nerves, and lacked an intact vas deferens did not impede their ability to develop and sustain the definitive secondary sexual anatomy of the male species.
6.3 Phenotypic Outcomes in Group 3 (Allografts)
The somatic results derived from Cohort 3 (Birds 5 and 6)—the cockerels that had undergone reciprocal cross-transplantation of allogeneic testes—provided the ultimate, breathtaking validation of Berthold’s hypothesis. Had allogeneic tissue transplantation failed due to immediate immunological rejection or required some hyper-specific neuro-somatic compatibility between an individual and its own organs, Birds 5 and 6 would have inevitably slipped into the atrophic, unmasculinized phenotype displayed by the Cohort 1 capons. Instead, both birds mirrored the dramatic virilization observed in the autografted subjects of Cohort 2.
Birds 5 and 6 exhibited full, vigorous development of their combs and wattles, with the cranial appendages displaying intense scarlet vascularity, profound structural elasticity, and prominent adult dimensions. Their plumage underwent identical masculine maturation, developing the elongated, glistening hackles, lustrous saddle plumage, and sweeping tail sickles typical of dominant males. They exhibited full muscular development, sharp tarsometatarsal spurs, and the proud, upright, vigilant posture of prime roosters. The phenotypic rescue was total: the foreign testicular graft derived from an unrelated individual and deposited into an alien peritoneal cavity functioned with the exact same somatic efficacy as an autologous organ. Under identical housing and environmental conditions, Cohorts 2 and 3 stood in stark, magnificent physical contrast to the pale, atrophied, lethargic capons of Cohort 1, demonstrating unequivocally that the testicular stimulus was universal, systemic, and utterly indifferent to anatomical locus.
7. Behavioral Modifications, Ethology, and Secondary Sexual Phenotypes
7.1 Vocalizations and Auditory Markers
While the somatic and morphological divergences among the three cohorts were visually undeniable, the behavioral and ethological transformations recorded by Berthold were even more profound. Among the most prominent behavioral markers of the adult domestic rooster is its distinctive vocal repertoire, particularly the territorial crow (cock-a-doodle-doo). Crowing is not merely an auditory display; it is an instinctual, testosterone-driven behavioral program utilized to announce territorial boundaries, reinforce social hierarchy, and communicate reproductive fitness across vast physical distances. The performance of this vocalization requires the coordinated activation of specialized syringeal musculature, enhanced respiratory dynamics, and a high degree of central nervous system arousal.
In Cohort 1, the castrated controls suffered a complete and permanent loss of this vocal display. Birds 1 and 2 never crowed. Over the entire duration of the post-operative observation period, their vocalizations were restricted to quiet, low-frequency clucks, food murmurs, and plaintive, juvenile alarm chirps typical of hens or immature chicks. They were entirely silent in the early dawn hours, lacking the neural motivation and syringeal turgor required to produce the triumphant, ringing call of the rooster.
Conversely, the birds in Cohort 2 and Cohort 3 retained and fully expressed their masculine vocal capacity. Birds 3, 4, 5, and 6 began to crow at precisely the developmental epoch when intact male cockerels achieve auditory maturity. Their crows were loud, clear, resonant, and acoustically robust, executed with the classic postural display: stretching the neck vertically, flapping the wings, elevating the thoracic cage, and projecting the sound across the farmyard. Berthold demonstrated that this complex, neuro-muscular vocal behavior was not driven by direct innervation between the brain and the native testes; rather, the neural centers of the avian brain governing the vocalization reflex were being continuously stimulated, organized, and activated by a blood-borne mediator originating from the ectopically grafted gonads.
7.2 Aggression, Dominance Hierarchies, and Territoriality
Equally striking was the profound divergence in social temperament, agonistic behavior, and dominance hierarchy dynamics across the experimental cohorts. Domestic roosters are notoriously combative animals that establish rigid, linear “pecking orders” through physical aggression, displays of physical stature, and violent combat involving their beaks, wings, and tarsometatarsal spurs. An intact rooster will ferociously defend his territory against rival males and maintain absolute social dominance over his domestic flock.
The castrates of Cohort 1 exhibited a complete collapse of social assertiveness. Birds 1 and 2 were utterly passive, meek, and docile. When placed into the communal yard among other birds, they actively retreated from the slightest confrontational gesture, yielding their position without resistance. They never initiated an agonistic encounter, never engaged in territorial disputes, and were continuously relegated to the lowest, most degraded tier of the avian social hierarchy. They lived in a state of behavioral neutrality, wandering aimlessly through the perimeter of the enclosure, indifferent to the presence of rival males and unmotivated to carve out a territory of their own.
In sharp contrast, the birds bearing autografts (Cohort 2) and allografts (Cohort 3) displayed the ferocious, unyielding aggression characteristic of prime adult cocks. Birds 3, 4, 5, and 6 engaged in vigorous, prolonged male-male combat to establish and defend their social status. They flared their neck hackles, executed aggressive ground-pecking displays, engaged in violent aerial spurring clashes, and pursued subordinate birds with unrelenting vigor. They patrolled the enclosures with vigilant, dominant postures, actively defending designated physical territories and establishing stable, linear pecking orders among themselves and other intact roosters. The presence of the translocated, non-innervated testes in their peritoneal cavities was fully sufficient to preserve the complex neural pathways, hypothalamic circuits, and motor patterns that mediate instinctual territorial aggression.
7.3 Reproductive Drive and Courtship Rituals
Perhaps the most fascinating ethological observation made by Berthold was the absolute preservation of male sexual drive, courtship rituals, and copulatory behaviors in the grafted birds, set against its total extinction in the castrated controls. The courtship ritual of Gallus gallus domesticus is an intricate, highly stereotyped behavioral sequence: an interested rooster approaches a receptive hen, lowers one wing to the ground, and executes a rhythmic, circular lateral shuffle known as the “courtship waltz” (or tidbitting display). If the hen crouches in sexual receptivity, the rooster mounts her back, grasps the plumage of her nape with his beak for stabilization, treads upon her dorsum, and brings his cloaca into apposition with hers to effect the “cloacal kiss” and transfer semen.
In Cohort 1, this entire behavioral matrix was completely absent. The capons displayed complete sexual anhedonia and reproductive apathy. They ignored estrous hens completely, never executed courtship waltzes, never vocalized courtship food calls, and never attempted to mount or copulate with females, treating them with the same indifferent detachment they exhibited toward other capons. Their reproductive instinct had been completely extinguished by the ablation of their gonads.
Cohorts 2 and 3, however, demonstrated intense, persistent, and successful male reproductive drive. Birds 3, 4, 5, and 6 actively pursued hens throughout the yard, performed flawless courtship waltzes, vocalized enticing nesting and food calls, and aggressively mounted and trod receptive females. Their copulatory mechanics were executed with vigor and anatomical precision. Yet, here lay one of the most intellectually illuminating paradoxes of Berthold’s experiment: because the testes of these birds were situated ectopically within the peritoneal cavity, their excretory ducts—the ductuli efferentes and the ductus deferens—had been severed and left completely disconnected from the cloaca. Consequently, when these grafted birds copulated, their ejaculates were entirely devoid of spermatozoa; they were functionally and physically sterile.
Berthold had uncovered a critical conceptual distinction: he demonstrated that sexual desire, instinctive mating behavior, and physical virility are physiologically dissociated from the mechanical capacity to transport sperm. The testes do not stimulate sexual drive via the physical passage of seminal fluid through internal nervous conduits; rather, the urge to mate is stimulated independently by an endocrine signal that acts directly upon the central nervous system, proving that libido is a humoral phenomenon rather than a mechanical or exocrine one.
8. Berthold’s Deductive Leap: The Blood-Borne Secretion Hypothesis
8.1 Post-Mortem Dissection and Anatomical Discoveries
After several months of continuous observation had verified the stability of these somatic and ethological phenotypes, Berthold terminated the experiment to perform comprehensive post-mortem dissections on all six cockerels. The primary objective of these necropsies was to ascertain the precise anatomical fate of the transplanted organs: Had the grafted testes survived, or had they withered away, leaving the birds to run on some form of persistent physiological momentum? And if they had survived, what physical connections had they formed with the host organism?
Dissection of the Cohort 1 capons confirmed the absolute success of the primary surgical ablations: the sub-lumbar retroperitoneal space was completely devoid of testicular tissue, with no evidence of accidental remnants or parenchymal regeneration. When Berthold opened the abdominal cavities of the birds in Cohort 2 and Cohort 3, he made an astonishing anatomical discovery. In all four birds that had retained their masculine secondary sexual characteristics (Birds 3, 4, 5, and 6), the transplanted testes had not withered, necrosed, or been absorbed. Instead, the organs had established robust, permanent biological anchors within the peritoneal cavity.
The ectopic testes had adhered firmly to the serosal surface of the small intestines, nestled among the folds of the mesenteric peritoneum. Most remarkably, Berthold observed that the grafted gonads were thoroughly engorged with blood. The host’s mesenteric vasculature had undergone intense, spontaneous neo-vascularization: microscopic capillaries and macroscopically visible blood vessels had sprouted from the intestinal serosa, breached the tunica albuginea of the testes, and arborized extensively throughout the internal glandular parenchyma. Upon incising the translocated testes, Berthold observed healthy, viable, pale-yellow seminiferous tissue that bled freely, confirming that the organs were richly perfused by the systemic circulatory system of the host. In some of the transplanted testes, he even observed microscopic evidence of ongoing, active spermatogenesis within the convoluted tubules, proving that the testicular tissue remained fully functional despite its bizarre anatomical location.
8.2 Falsification of the Neural Regulation Theory
The definitive intellectual crisis for the reigning neurocentric paradigm occurred when Berthold meticulously examined the ectopic grafts for evidence of neural connections. Armed with the finest anatomical dissection instruments and magnifying lenses of his day, he scrutinized the tissue interfaces connecting the translocated testes to the host’s intestinal walls and mesentery. His findings were absolute: there was not a single nerve fiber connecting the transplanted testes to the host nervous system.
The original spermatic nerves branching from the sympathetic chains and aortic plexuses had been severed during the initial castration and remained dead, retracted stumps in the empty sub-lumbar fossa. The ectopic sites of implantation—the serosal surfaces of the jejunum and ileum—possessed only the delicate intrinsic enteric innervation (Auerbach’s and Meissner’s plexuses) and autonomic vagal/sympathetic fibers dedicated to gastrointestinal motility and vasomotor tone. None of these pathways had sprouted, penetrated, or established structural synaptic contacts with the parenchymal cells of the grafted gonads. The organs were completely, unequivocally denervated.
This single, irrefutable observation systematically falsified the neural regulation theory. If the testes were entirely devoid of neural connections to the spinal cord, sympathetic trunks, or brain, it was physically and logically impossible for them to transmit electrical impulses, reflex signals, or neural vibrations to the rest of the body. The nervous system could no longer be championed as the obligate conduit through which the gonads sustained the growth of the comb, maintained the plumage, prompted crowing, or ignited the fires of territorial aggression and sexual libido. The centuries-old paradigm had reached its definitive empirical limit; the biological effect was real, but the hypothesized neural mechanism was demonstrably dead.
8.3 The Concept of Blood-Mediated Humoral Action
Confronted with the collapse of the neurocentric explanation, Berthold executed the brilliant deductive leap that marked the birth of modern endocrinology. In his 1849 paper, he synthesized his anatomical, somatic, and behavioral findings into a revolutionary conclusion that shattered existing physiological dogmata. Berthold reasoned with impeccable inductive logic: if the transplanted testes maintain the masculine phenotype without possessing any connection to the nervous system, and without possessing any functional excretory ducts to discharge external fluids, their systemic influence can only be exerted through one remaining medium—the circulating blood.
Berthold posited that the testes alter the blood as it courses through their richly vascularized internal capillary beds, and that this blood, dynamically modified by its passage through the organ, is propelled throughout the systemic vascular network to act upon distant target tissues. In his own historic words, translated from the German:
“The testes act upon the blood, and the blood acts upon the whole organism, of which, however, the nervous system forms a very essential part.”
This formulation was astonishing in its conceptual maturity. Berthold did not simply revive ancient humoralism; he transformed it. Rather than speaking of mystical balance among four classical bodily fluids, he advanced a modern, mechanistic, vascular-borne physiological theory. He proposed that an isolated organ releases a specific functional property or material substance into the bloodstream, which then circulates systemically to modify both physical structures (the comb, wattles, and plumage) and central neural circuits (instincts, vocal reflexes, and reproductive drive). Berthold had formulated the foundational concept of what the French physiologist Claude Bernard would later describe as the “internal secretion” (sécrétion interne), establishing the bloodstream as a high-speed vehicle for targeted chemical communication within the multicellular animal.
9. Contemporary Reception and Mid-Nineteenth-Century Obscurity
9.1 The 1849 Göttingen Royal Scientific Society Presentation
On February 10, 1849, Berthold formally presented the preliminary results of his investigation before the prestigious Royal Scientific Society of Göttingen (Königliche Societät der Wissenschaften zu Göttingen). The presentation was delivered to an audience of esteemed German academics, anatomists, and philologists who respected Berthold as an accomplished naturalist and comparative anatomist. Shortly thereafter, his concise, four-page paper, “Transplantation der Hoden,” appeared in Johannes Müller’s Archiv für Anatomie, Physiologie und wissenschaftliche Medicin. The journal was arguably the most prestigious physiological periodical in the German-speaking world, ensuring that Berthold’s text was physically distributed to major university libraries across Europe.
Yet, despite its premier placement and the profound implications of its conclusions, the paper was met with near-total silence. There was no widespread intellectual uproar, no flurry of frantic replications, and no immediate recognition that a new biological epoch had dawned. The report was largely treated as a curious, marginal anatomical anecdote regarding the remarkable survival properties of avian tissues. Following this single publication, Berthold himself mysteriously stepped back from this line of investigation. He did not publish another word on testicular transplantation, secondary sexual traits, or the circulatory properties of organs for the remainder of his life. He redirected his intellectual energies toward his duties as curator of the zoological collections, publishing treatises on reptiles, amphibians, and lice, and teaching conventional physiology until his death in Göttingen on January 3, 1861, wholly unaware of the revolutionary canonization his modest experiment would posthumously receive.
9.2 Reasons for Delayed Scientific Integration
The prolonged, fifty-year dormancy of Berthold’s findings serves as a classic case study in the sociology of scientific knowledge and paradigm resistance, as articulated by Thomas Kuhn. Why did the European scientific establishment fail to recognize the magnitude of the 1849 experiment? The primary reason was the sheer, overwhelming momentum of the neurophysiological research program. The mid-nineteenth century was the golden age of electrophysiology and reflex arc mapping. Brilliant, charismatic figures such as Emil du Bois-Reymond, Hermann von Helmholtz, Carl Ludwig, and Claude Bernard were systematically demonstrating that nerves controlled muscle contractions, regulated heart rate, and modulated glandular exocrine secretions through vasomotor tone. In this intoxicating intellectual atmosphere, the scientific community was thoroughly committed to finding neural explanations for all biological phenomena; Berthold’s blood-borne hypothesis was a conceptual anomaly that fit nowhere within the reigning paradigm.
Furthermore, mid-nineteenth-century chemistry was completely incapable of isolating, characterizing, or even conceptualizing the minute chemical quantities at play. Organic chemistry was in its infancy; the concepts of trace biomolecules, receptors, and steroidal structures were far beyond the methodological horizon. To hypothesize that a microscopic chemical entity released in trace amounts by a pair of testes could orchestrate the growth of a rooster’s comb seemed dangerously close to the unscientific, vitalistic speculations that nineteenth-century laboratory medicine was actively trying to eradicate. Without the analytical chemistry needed to isolate the active substance from the blood, Berthold’s “action upon the blood” remained an abstract, unprovable black box.
Finally, the medical world lacked an overarching conceptual framework to link Berthold’s isolated avian findings to other scattered clinical observations. Conditions such as diabetes mellitus, Addison’s disease (described in 1855), and myxedema were viewed as separate, unrelated pathologies of unknown etiology, rather than as manifestations of a unified systemic endocrine network. Berthold’s paper was read by anatomists simply as an interesting demonstration of tissue grafting—a field of minor plastic surgical curiosity—rather than as the revelation of an entirely new biological communication system. As a result, the paper slipped quietly beneath the waves of physiological literature, forgotten by all but a handful of obscure compilers.
9.3 Early Replications and Competing Theories
During the decades of dormancy between 1850 and the late 1880s, a small number of European researchers attempted sporadic replications or variations of avian gonadal transplantation, often with deeply frustrating and contradictory results. Operating in the pre-antiseptic era, many of these investigators encountered catastrophic post-operative mortality rates: their birds routinely succumbed to acute bacterial peritonitis, sepsis, or internal hemorrhage. In other instances, technical failures abounded: ectopically deposited testes frequently underwent ischemic liquefactive necrosis, became walled off as calcified, non-functional fibrous nodules, or were rapidly absorbed by the host’s peritoneal macrophages.
Conversely, some researchers who attempted castration failed to achieve complete removal of the native testes, leaving microscopic fragments of the tunica albuginea and seminiferous tissue adhered to the vena cava. These fragments underwent massive compensatory hypertrophy, restoring the masculine phenotype and misleading the investigators into believing that non-gonadal tissues or incomplete surgeries were maintaining virility. Competing theoretical explanations were eagerly advanced to explain away any positive results without abandoning the beloved neurocentric model. Skeptics argued that transplanted testes did not alter the blood chemically; rather, they suggested that the physical presence of the graft acted as a non-specific mechanical irritant to the peritoneal sympathetic plexuses, triggering a diffuse, generalized reflex that stimulated the bird’s metabolism.
Others proposed purely “nutritive” hypotheses, suggesting that the gonads merely absorbed certain toxic impurities from the blood, acting as passive biological filters rather than active secretory organs. Because these disparate investigators lacked standardized surgical environments, uniform genetic lines of animals, and any means to quantify hormonal concentrations in the serum, their conflicting results muddled the literature. Berthold’s 1849 experiment was relegated to the status of an anomalous, irreproducible nineteenth-century report, an isolated historical oddity awaiting a clinical and intellectual catalyst to rescue it from obscurity.
10. The Re-Discovery of Berthold’s Work and the Rise of Modern Endocrinology
10.1 Charles-Édouard Brown-Séquard and Organotherapy
The historical catalyst that shattered this decades-long dormancy and dramatically resurrected Berthold’s work occurred on June 1, 1889, in Paris. On that day, the renowned 72-year-old Franco-American physiologist Charles-Édouard Brown-Séquard delivered a sensational address before the Société de Biologie. Brown-Séquard announced that he had successfully rejuvenated his own failing physical vigor, mental acuity, and muscular strength through a series of self-administered subcutaneous injections. The substance he injected was an aqueous extract derived from the ground-up testicles of freshly slaughtered dogs and guinea pigs.
Brown-Séquard claimed that this crude organic elixir—which quickly captured the popular imagination as the “Elixir of Life”—restored his youthful stamina, increased the distance of his urinary arc, relieved his chronic constipation, and amplified his intellectual capacity. While modern biochemistry has definitively established that aqueous testicular extracts contain virtually zero hydrophobic testosterone, and that Brown-Séquard’s personal rejuvenation was almost certainly an extraordinary manifestation of the psychological placebo effect, the scientific shockwave of his announcement transformed European medicine. It ignited the global craze of “organotherapy”—the clinical administration of crude animal glandular extracts to treat human aging, debility, and disease.
In the wake of Brown-Séquard’s sensational claims, historians of medicine and physiologists began scouring the older scientific literature to discover who had first empirically demonstrated the systemic, rejuvenating, blood-mediated actions of the gonads. It was during this intense retrospective re-examination that German and French scholars pulled Johannes Müller’s 1849 Archiv from the library archives and rediscovered Berthold’s four-page masterwork. Berthold was suddenly transformed from an obscure mid-century Göttingen naturalist into an intellectual prophet. His controlled, surgically pristine experiment on cockerels provided the rigorous, empirical justification that Brown-Séquard’s controversial, self-experimenting organotherapy desperately needed, securely anchoring the burgeoning study of internal secretions in experimental soil.
10.2 Serge Voronoff and Gonadal Xenotransplantation
The intellectual momentum generated by the rediscovery of Berthold’s work, combined with the sensationalism of organotherapy, culminated during the 1920s in one of the most controversial chapters in biomedical history: the xenotransplantation craze orchestrated by the Russian-French surgeon Serge Voronoff. Operating at the prestigious Collège de France in Paris, Voronoff traced his direct ideological and surgical lineage straight back to Berthold’s 1849 protocol. He reasoned that if an ectopic testicular graft could vascularize and fully masculinize a castrated cockerel, then surgically grafting testicular tissue into aging human males could reverse the senescence of the human body.
Lacking a sufficient supply of human testicular donors, Voronoff turned to non-human primates. He harvested thin, transverse slices of testes from chimpanzees and baboons, surgically grafting them directly into the tunica vaginalis of elderly men suffering from physical debility, memory decline, and loss of sexual libido. Voronoff argued that the primate grafts would establish microvascular anastomoses with the scrotal tissues, discharging their vital internal secretions directly into the patient’s bloodstream. For nearly a decade, Voronoff was celebrated as an international scientific celebrity, performing hundreds of “monkey gland” transplants on wealthy European and American elites who reported miraculous, albeit temporary, revivals of vigor and sexual potency.
Inevitably, the entire Voronoff enterprise collapsed under the relentless realities of transplant immunology and pathology. Because the mammalian immune system fiercely rejects allogeneic and xenogeneic tissue barriers, the primate grafts were systematically attacked by human lymphocytes, undergoing acute ischemic necrosis, liquefactive degeneration, and dense fibrous scarring within weeks or months. By the late 1920s, rigorous clinical follow-ups demonstrated that Voronoff’s patients experienced no objective physiological longevity benefits, leading the established medical community to condemn the procedure as expensive, dangerous quackery. Yet, from an epistemological perspective, Voronoff’s audacious surgical campaign had an unexpected, positive consequence: it generated massive institutional, commercial, and academic pressure to identify, isolate, and chemically synthesize the true, molecular active principle of the testes, shifting the focus of endocrine research away from dangerous tissue transplantations and directly into the domain of pure analytical biochemistry.
10.3 Bayliss, Starling, and the Coining of the ‘Hormone’
While surgeons were experimenting with tissue grafts, the basic physiological mechanisms of non-neural bodily coordination were finally achieving undeniable molecular confirmation. The watershed moment arrived in 1902 at University College London, through the collaborative investigations of English physiologists William Maddock Bayliss and Ernest Henry Starling. Investigating the regulatory mechanisms governing pancreatic exocrine secretion, Bayliss and Starling severed all autonomic and mesenteric nerves supplying a looped segment of a dog’s jejunum, leaving the bowel connected to the body solely by its arterial and venous blood vessels.
When they introduced dilute hydrochloric acid into the denervated intestinal lumen to simulate the passage of gastric chyme from the stomach, the pancreas—which had no physical or nervous contact with the intestinal loop—immediately responded by pouring out copious volumes of alkaline digestive juice. Bayliss and Starling realized that the acid had acted upon the epithelial cells of the intestinal mucosa, liberating a chemical substance into the local venules. This substance coursed through the portal circulation, traversed the systemic heart and lungs, and arrived at the pancreas via the pancreatic arteries, stimulating robust secretory activity. They extracted this substance from the intestinal mucosa, injected it intravenously into a healthy dog, and observed immediate pancreatic secretion. They named this chemical messenger secretin.
In 1905, delivering the prestigious Croonian Lectures before the Royal College of Physicians in London, Starling recognized that secretin was not an isolated biological curiosity; it represented an entirely new, universal class of physiological regulators. Consulting with the Cambridge classicist W. T. Vesey, Starling introduced a new term to the biological lexicon: hormone, derived from the classical Greek verb ὁρμάω (hormao), meaning “to excite,” “to arouse,” or “to set in motion.” Starling defined a hormone as a chemical substance produced by an organ or tissue in small quantities, which is discharged directly into the circulating blood and carried to distant target organs, where it evokes specific, functional physiological responses.
With the coining of the word “hormone” and the establishment of endocrinology as a distinct biomedical discipline, the scientific world looked back across the preceding half-century and formally canonized Arnold Adolph Berthold. His 1849 rooster testicle experiment was universally recognized as the pioneering, foundational demonstration of a hormone in action. Berthold had discovered the functional reality of internal secretion fifty-six years before Starling provided the modern conceptual terminology to describe it.
11. The Biochemical Trajectory: From Testicular Substance to Testosterone
11.1 The Rooster Comb Bioassay (The Capon Unit)
Armed with the conceptual framework of hormones, twentieth-century biomedical science confronted the daunting challenge of isolating the specific chemical molecule that Berthold had demonstrated was emanating from the rooster’s testes. However, early twentieth-century biochemists faced a massive methodological barrier: they could not see, measure, or purify an unknown substance from crude biological fluids without an objective, quantitative tool to verify its presence and concentration. In an extraordinary historical loop, the scientific community turned directly back to Berthold’s original animal model—the castrated cockerel—to forge the primary analytical instrument of androgen discovery: the rooster comb bioassay.
Developed and standardized in the late 1920s by physiological chemists such as Carl R. Moore and Fred C. Koch at the University of Chicago, the capon comb growth bioassay transformed Berthold’s qualitative, observational study into a rigorous, highly sensitive pharmacological metric. Young cockerels were surgically castrated to produce standardized, atrophic capon combs. These birds then served as living biological test tubes. Researchers extracted lipid and aqueous fractions from mammalian testes or human urine, dissolved them in organic solvents or oils, and applied them either systemically via intramuscular injection or topically by directly rubbing the extract onto the shriveled, pale comb of the capon.
If the chemical extract contained the active androgenic principle, the comb responded with astonishing sensitivity: within days, the cells of the comb’s dermis began proliferating, the rich capillary vascular networks engorged with blood, and the tissue expanded in surface area and vertical height. Biochemists meticulously measured the increase in the comb’s length and height using precision calipers, or recorded the expansion of its silhouette using photographic shadow-projections. Koch and his colleagues established the international “Capon Unit” (defined as the minimum amount of a substance that, when administered daily for five consecutive days to a standardized capon, induced an increase of exactly 20% in the surface area of the comb). Through this direct methodological evolution from Berthold’s 1849 observations, the avian comb provided the indispensable bioassay that guided chemists through every step of extraction, purification, and isolation.
11.2 Isolation and Crystallization of Androsterone and Testosterone
With the capon comb bioassay serving as an infallible detection compass, the global race to chemically isolate the pure testicular hormone exploded into a fiercely competitive pursuit. In 1931, the brilliant German biochemist Adolf Butenandt, working at the University of Göttingen—the very same academic institution where Berthold had performed his experiments eighty-two years earlier—achieved the first major chemical breakthrough. Recognizing that testicular hormones or their metabolites were excreted in mammalian urine, Butenandt collaborated with the chemical firm Schering-Kahlbaum to process an astonishing 25,000 liters of urine collected from Berlin police barracks.
Through massive, industrial-scale liquid-liquid extraction, steam distillation, and chromatographic fractionation, Butenandt succeeded in isolating a minuscule 15 milligrams of a pure, crystalline substance that triggered explosive comb growth in the capon bioassay. He elucidated its empirical formula, demonstrating that it was a steroid molecule, and named it androsterone. However, physiologists quickly noted a biochemical discrepancy: pure androsterone, while unquestionably potent in stimulating the capon comb, was significantly less active per milligram than crude lipid extracts harvested directly from mammalian testicular tissue. This proved that androsterone was merely a weakened metabolic degradation product excreted by the kidneys, and that the true, primary gonadal hormone remained locked within the testicular parenchyma.
The ultimate triumph occurred in May 1935 in Amsterdam. A research team led by the pharmacologist Ernst Laqueur at the University of Amsterdam processed nearly one metric ton of freshly harvested bull testes. Utilizing organic solvent extractions and low-temperature crystallization, Laqueur’s group successfully isolated 10 milligrams of a pure, glistening, white crystalline steroid. This primary hormone was extraordinarily potent, displaying a pharmacological activity in the capon comb bioassay hundreds of times greater than Butenandt’s androsterone. In a historic paper published in June 1935, Laqueur christened this newly discovered hormone testosterone (derived from testis, sterol, and the suffix -one, denoting its chemical ketone group).
Within months of Laqueur’s isolation, Adolf Butenandt in Germany and the Croatian-Swiss chemist Leopold Ruzicka in Zurich independently announced the full chemical synthesis of testosterone from cholesterol, an intellectual and industrial tour de force that earned both Butenandt and Ruzicka the Nobel Prize in Chemistry in 1939. Eighty-six years after Berthold had posited that an unknown property was released by the translocated testes into the circulating blood of six cockerels, the exact, three-dimensional molecular architecture of that substance had finally been isolated, crystallized, synthesized, and structurally mapped.
11.3 Modern Characterization of the Hypothalamic-Pituitary-Gonadal Axis
Today, our understanding of the physiological phenomenon Berthold observed extends into the deepest molecular recesses of cellular genomics and neuroendocrinology. We now recognize that the testes do not act as autonomous, isolated endocrine islands; rather, they operate as the peripheral executive units of an intricate, multi-tiered, closed-loop regulatory feedback network known as the Hypothalamic-Pituitary-Gonadal (HPG) axis.
The operational cascade begins in the central nervous system, within the arcuate and preoptic nuclei of the hypothalamus. Specialized neurosecretory neurons synthesize and release pulsatile bursts of Gonadotropin-Releasing Hormone (GnRH). This decapeptide traverses the hypophyseal portal microvasculature to bind with high affinity to G-protein coupled receptors on the surface of gonadotrope cells in the anterior pituitary gland. The pituitary responds by synthesizing and releasing two powerful dimeric glycoprotein hormones into the systemic circulation: Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH).
Upon reaching the avian or mammalian gonads, LH binds specifically to receptors on the interstitial Leydig cells, activating the cyclic adenosine monophosphate (cAMP) and protein kinase A (PKA) intracellular signaling cascades. This enzymatic cascade mobilizes cholesterol from cellular storage, transferring it across the outer and inner mitochondrial membranes via the Steroidogenic Acute Regulatory (StAR) protein. Inside the mitochondria and smooth endoplasmic reticulum, cholesterol undergoes a series of precise enzymatic cleavages—mediated by cytochrome P450 enzymes (including CYP11A1 and CYP17A1) and 17β-hydroxysteroid dehydrogenase (17β-HSD)—to yield testosterone. Concurrently, FSH acts upon the Sertoli cells of the seminiferous tubules, promoting active spermatogenesis, the very cellular maturation Berthold observed in his post-mortem histological examinations of the vascularized intestinal grafts.
Once synthesized, testosterone—a lipophilic molecule—diffuses freely across the cell membrane into the capillary bloodstream, precisely as Berthold deduced. It circulates throughout the body bound primarily to carrier proteins such as sex hormone-binding globulin (SHBG) and serum albumin. Upon reaching target tissues such as the fleshy dermal layers of the avian comb, the specialized syringeal vocal musculature, and the neurobehavioral circuits of the preoptic area of the anterior hypothalamus, testosterone diffuses into target cells. In many peripheral tissues, it is enzymatically converted by the enzyme 5α-reductase into dihydrotestosterone (DHT), an androgen of even greater potency, or by the enzyme aromatase (CYP19A1) into estradiol, which mediates critical behavioral and neuroplastic actions within the brain.
Within target cells, testosterone or DHT binds to the intracellular androgen receptor (AR), triggering the dissociation of heat shock proteins, structural dimerization of the receptor, and rapid translocation of the hormone-receptor complex into the cell nucleus. There, the complex binds to specific, consensus DNA sequences known as Androgen Response Elements (AREs) located in the promoter and enhancer regions of target genes. This molecular event recruits co-activators and RNA polymerase II, modulating the transcription of hundreds of structural proteins, growth factors, and angiogenic mediators (such as Vascular Endothelial Growth Factor, or VEGF). In the rooster’s comb, this genomic activation stimulates rapid cellular hyperplasia, profound extracellular matrix accumulation of hyaluronic acid, and intense capillary neo-angiogenesis, generating the turgid, bright-red, vascular ornaments Berthold observed. In the central nervous system, it remodels synaptic architecture and neurochemical sensitivity, driving the instinctual motor programs of territorial aggression, morning crowing, and the courtship waltz. Systemic testosterone concentrations are maintained in a tight, homeostatic steady state via negative feedback, wherein circulating testosterone suppresses the transcription and secretion of GnRH in the hypothalamus and LH in the anterior pituitary. Through this closed-loop architecture, modern science has achieved a total molecular resolution of the physiological miracle that Berthold observed macroscopically on his farmyard cockerels in 1849.
12. Lasting Legacy and Epistemological Impact on Modern Biomedical Science
12.1 Methodological Paradigms: Ablation and Replacement
Beyond its direct parentage of reproductive endocrinology, Arnold Adolph Berthold’s 1849 experiment exerted a permanent epistemological impact upon the methodological architecture of all modern biomedical science. Berthold established what has universally become known as the classic endocrine experimental paradigm, an indispensable three-step logical protocol:
- Surgical Ablation: The targeted removal of a suspected physiological organ or tissue, followed by the rigorous observation and documentation of the resulting somatic, metabolic, or behavioral deficit state.
- Replacement / Transplantation: The ectopic replacement, reimplantation, or systemic substitution of the ablated organ, tissue, or its biochemical extract, confirming that the intervention rescues the organism from its deficit state.
- Re-ablation / Negative Verification: Demonstrating that subsequent removal of the replaced organ reinstates the original deficit phenotype, verifying that the rescue was strictly dependent on the continued biological presence and activity of that specific tissue.
This simple yet profound deductive triad became the master blueprint for the heroic age of endocrinological discovery throughout the late nineteenth and twentieth centuries. It was the precise logical framework used by Moritz Schiff in 1884 to prove the endocrine function of the thyroid gland, by Joseph von Mering and Oskar Minkowski in 1889 to demonstrate that pancreatectomy produces diabetes mellitus, and by Frederick Banting and Charles Best in 1921 to isolate insulin from the pancreatic islets of Langerhans. The same ablation-replacement logic was deployed systematically to elucidate the essential functions of the adrenal cortex, the parathyroid glands, and the anterior pituitary.
In contemporary molecular biology, Berthold’s paradigm persists in its most advanced, sophisticated iterations. The modern techniques of functional reverse-genetics—specifically the construction of gene knockout models via homologous recombination or CRISPR-Cas9 genome editing, followed by genetic rescue through transgenic knockin or inducible expression systems—are the direct conceptual descendants of Berthold’s surgical ablation-replacement logic. Rather than excising an entire macroscopically visible gonad with a scalpel, the contemporary molecular biologist uses molecular scissors to ablate a single target gene, observes the developmental and phenotypic deficits in the cellular or organismal model, and subsequently restores the gene to confirm the rescue of normal biological function. Berthold fundamentally shifted biology from a purely descriptive, passive observational discipline into a functional, interventionist, and mechanistic experimental science.
12.2 Early Foundations of Transplantation Biology and Angiogenesis
While celebrated primarily within the annals of endocrinology, Berthold’s experiments simultaneously represent a seminal, often unacknowledged milestone in the history of transplantation biology and angiogenesis. In 1849, long before the complex immunological mechanisms of the Major Histocompatibility Complex (MHC), human leukocyte antigens (HLA), and T-cell mediated tissue rejection were understood, Berthold successfully executed both free autologous tissue grafts and allogeneic cross-transplantations within a vertebrate species.
Berthold was the first investigator to record the phenomenon of spontaneous, functional neo-vascularization supporting a free, detached parenchymal tissue graft. His post-mortem anatomical discovery—that the host’s intestinal and mesenteric blood vessels had actively sprouted, invaded the tunica albuginea, and established a microcirculatory capillary bed sufficient to sustain both the survival and the complex spermatogenic and steroidogenic functions of the transplanted gonads—anticipated the modern field of angiogenesis research championed by Judah Folkman in the late twentieth century. Berthold provided the first empirical proof that an organ does not require the microvascular continuity of its original embryonic pedicle to survive; rather, it possesses an intrinsic biological capacity to stimulate vascular sprouting from adjacent, non-native host tissues to ensure its own metabolic survival.
Furthermore, Berthold’s reciprocal cross-transplantation between cockerels in Cohort 3 demonstrated an exceptional degree of allogeneic tolerance within domestic avian species. While mammalian allografts typically undergo aggressive acute rejection due to divergent MHC Class I and Class II disparities, domestic chickens—particularly those derived from localized, inbred agricultural strains—exhibit unique immunological characteristics that occasionally permit sustained allogeneic graft acceptance. Berthold’s documentation of viable, active testicular tissue surviving for months in an unrelated host organism provided early surgical pioneers with empirical evidence that tissue and organ transplantation across individual biological barriers was an achievable physiological goal, laying the earliest historical groundwork for the emergence of modern reconstructive and transplant surgery.
12.3 Pedagogical and Historical Significance
In modern biomedical education, Arnold Adolph Berthold’s 1849 experiment occupies an exalted pedagogical position. It is universally cited in introductory undergraduate biology texts, medical school physiology lectures, and advanced endocrinology seminars as the supreme exemplar of experimental design. Its beauty resides in its parsimony, its intellectual clarity, and its extraordinary ratio of conceptual insight to economic expenditure. With a sample size of only six domestic birds, a simple surgical scalpel, silk sutures, and an open farmyard, Berthold formulated an experimental architecture that dismantled an erroneous, centuries-old neurocentric scientific consensus and inaugurated an entire biomedical discipline.
The history of Berthold’s work serves as a sobering, cautionary reminder of the non-linear, unpredictable trajectory of scientific progress. It demonstrates that profound, revolutionary truths can be discovered, documented, and published in the most prestigious academic journals of the day, only to be completely ignored, marginalized, and buried if the surrounding scientific community lacks the technological tools, the chemical methods, and the conceptual readiness to absorb them. Berthold’s half-century of obscurity illustrates the profound power of paradigm inertia; it reminds scientists that the absence of a known molecular mechanism does not invalidate the empirical reality of a rigorously observed biological phenomenon.
Today, as we manipulate the endocrine system with recombinant peptide hormones, designer steroidal ligands, small-molecule receptor antagonists, and targeted gene-therapy vectors, we trace our intellectual lineage directly back to that summer in Göttingen in 1849. Arnold Adolph Berthold saw beyond the anatomical cables of the nervous system, looking through the blood to discover the invisible, molecular dialogue that binds the disparate tissues of an animal body into a magnificent, harmonious, and unified living organism.
Conclusion
The rooster testicle transplant experiments executed by Arnold Adolph Berthold in 1849 stand as an enduring monument to the power of the experimental method in biological science. Confronting a medical establishment thoroughly committed to the neurocentric model of physiological coordination, Berthold possessed the intellectual audacity and surgical skill to ask whether an organ could govern the body through a non-neural, vascular medium. By castrating juvenile cockerels and transplanting their gonads to ectopic peritoneal sites devoid of native innervation, he demonstrated that secondary sexual ornaments, aggressive dominance hierarchies, territorial crowing, and instinctual courtship behaviors were preserved and governed not by electrical impulses traversing sympathetic nerve tracts, but by a functional property liberated directly into the circulating blood.
Though his brilliant four-page treatise languished in academic obscurity for nearly fifty years, the rediscovery of his work at the dawn of the twentieth century catalyzed the birth of modern endocrinology, directly inspiring the conceptual formulation of the hormone by Bayliss and Starling, the standardization of the capon comb bioassay, and the ultimate chemical isolation and synthesis of testosterone by Butenandt, Laqueur, and Ruzicka. Beyond the confines of reproductive biology, Berthold gifted modern science with the ablation-replacement experimental paradigm—a logical protocol that remains the gold standard of functional physiology and contemporary reverse-genetics. In the quiet, brilliant simplicity of his six Göttingen cockerels, Arnold Adolph Berthold broke the monopoly of the nervous system, revealing the vascular bloodstream as a dynamic highway of chemical communication and forever changing our understanding of the living organism.
References
- Bayliss, W. M., & Starling, E. H. (1902). The mechanism of pancreatic secretion. The Journal of Physiology, 28(5), 325–353. https://doi.org/10.1113/jphysiol.1902.sp000920
- Berthold, A. A. (1849). Transplantation der Hoden. Archiv für Anatomie, Physiologie und wissenschaftliche Medicin, 1849, 42–46. https://archive.org/details/archivfranatomi1849berl
- Borell, M. (1976). Brown-Séquard’s organotherapy and its appearance in America at the end of the nineteenth century. Journal of the History of Medicine and Allied Sciences, 31(3), 309–320. https://doi.org/10.1093/jhmas/xxxi.3.309
- Brown-Séquard, C. É. (1889). Des effets produits chez l’homme par des injections sous-cutanées d’un liquide retiré des testicules frais de cobaye et de chien. Comptes rendus de la Société de Biologie, 41, 415–419.
- Butenandt, A. (1931). Über die chemische Untersuchung der Sexualhormone. Zeitschrift für Angewandte Chemie, 44(46), 905–908. https://doi.org/10.1002/ange.19310444602
- David, K., Dingemanse, E., Freud, J., & Laqueur, E. (1935). Über krystallinisches männliches Hormon aus Hoden (Testosteron), wirksamer als aus Harn oder aus Cholesterin bereitetes Androsteron. Hoppe-Seyler’s Zeitschrift für physiologische Chemie, 233(5–6), 281–282. https://doi.org/10.1515/bchm2.1935.233.5-6.281
- Forbes, T. R. (1949). Arnold Adolph Berthold and the first endocrine experiment. Bulletin of the History of Medicine, 23(3), 263–277.
- Gallagher, T. F., & Koch, F. C. (1929). The testicular hormone. The Journal of Biological Chemistry, 84(2), 495–500. https://doi.org/10.1016/S0021-9258(18)77007-8
- Jorgensen, C. B. (1971). John Hunter, A. A. Berthold, and the origins of endocrinology. Acta Historica Scientiarum Naturalium et Medicinalium, 24, 1–54.
- Klein, R. (2015). A historical perspective on the discovery of the first hormone: Berthold’s rooster experiment. European Journal of Endocrinology, 173(4), R135–R141.
- Medvei, V. C. (1982). A History of Endocrinology. MTP Press. https://doi.org/10.1007/978-94-009-7304-6
- Moore, C. R., & Price, D. (1932). Gonad hormone functions, and the reciprocal influence between gonads and hypophysis with its bearing on the problem of sex-hormone antagonism. American Journal of Anatomy, 50(1), 13–71. https://doi.org/10.1002/aja.1000500103
- Ruzicka, L., & Wettstein, A. (1935). Über die künstliche Herstellung des Testikelhormons Testosteron (Androsten-3-on-17-ol). Helvetica Chimica Acta, 18(1), 1264–1275. https://doi.org/10.1002/hlca.193501801176
- Starling, E. H. (1905). The chemical regulation of the secretory process. Proceedings of the Royal Society of London. Series B, 76(512), 305–320. https://doi.org/10.1098/rspb.1905.0024
- Tata, J. R. (2005). One hundred years of hormones. EMBO Reports, 6(6), 490–496. https://doi.org/10.1038/sj.embor.7400444