PHYSIOLOGY, CHEMISTRY AND PATHOLOGY 439 



further development and functionation only in their presence. How- 

 ever, I would not leave the impression that any tissue in itself is sufficient 

 to establish sex. The sex of the subsequent individual, in all probability, 

 is determined at the time of fertilization, but, in certain animals, the 

 fullest degree of maleness or femaleness is assured only by the presence 

 of the reproductive tissue laid down in the embryo. 



3. The Relationship Between the Interstitial Cells of the- Testes 

 and the Secondary Sexual Characters. The question now arises, how 

 do the sex glands exert their influence upon the development and main- 

 tenance of somatic structures dependent upon them ? According to the 

 older physiologists this is accomplished by influences exerted through 

 the nervous system (Nussbaum). However, the results obtained by the 

 transplantation of gonadal tissue into previously castrated animals prove 

 this theory untenable. Numerous observations both on man and animals 

 have established the fact that the effects following gonadectomy are, to a 

 great extent, obviated by successful transplantation of gonadal tissue. 

 Preparations and extracts of reproductive tissue serve a similar purpose. 

 Obviously, therefore, the gonads exert their influence by means of sub- 

 stances liberated into the somatoplasm. Hence the testes may be consid- 

 ered as organs of internal secretion. The exact manner in which the 

 gonads exert their influence upon the somatic structures, however, remains 

 to be demonstrated. 



Castration necessitates the complete removal of the primary repro- 

 ductive organs ;' hence, results obtained by such procedures may be inter- 

 preted as due either to the loss of spermatogenic tissue, or other structures 

 present in the testes. Therefore, a study of the effects of castration 

 simply locates the organ, the presence of which is so necessary to the 

 proper development of masculinity. Observations upon the effects of 

 testicular transplants and the administration of extracts are open to the 

 same criticism. Such observations, therefore, throw no immediate light 

 upon the solution of the present problem. 



At present there is no evidence that the secondary sex characters are 

 in any way associated with the activities of the sperm-producing cells. On 

 the contrary, there is a wealth of evidence which indicates that these char- 

 acters are dependent upon certain cells lying in the interstices between 

 the seminiferous tubules, viz., the Interstitial Cells. 



Histology of the Interstitial Cells of the Testis. The interstitial cells 

 of the testis were first described by Leydig in 1850, and are commonly 

 referred to under his name. He characterized these structures as clear, 

 round cells, analogous to embryonic connective tissue, vacuolated and con- 

 taining fat and pigment granules. Kolliker in 1854 also described these 

 cells and further demonstrated their presence, not only in the interstices 

 of the seminiferous tubules, but also in the mediastinum and connective 

 tissue septa of the testis, and under the tunica albuginea. 



