136 Embryonic Development of Ovary and Testis of Mammals 



them even in the brain region of the young Teleost (Micrometrus). 

 There seems to be no doubt of their sexual character, because they are 

 present in such great numbers in the sex glands, and have all the char- 

 acteristics of those cells (spermatogonia and oogonia) from which the sex 

 products eventually form. 



It has been shown in the pig and rabbit, however, that these sex cells, 

 appearing in the indifferent stages, do not contribute to the formation of 

 functional sex products in the ovary. The same is probably true of the 

 testis, it being at least certain that the great bulk of the spermatogonia 

 are formed from the germinative cells — cells derived originally from the 

 peritoneum and maintaining, first, their indifferent character at least so 

 far as our technique is able to show. Schonfeld, oi, and Loisel, oo, 

 have shown, the former in the case of mammals, the latter in birds, that 

 spermatogonia and Sertoli i an cells arise during adult life from these 

 germinative cells (indifferent cells of Schonfeld). What interpretation 

 shall we then put upon the so-called primitive sex cells (primitive ova, 

 XJreier, Urkeimzellen, etc.) ? I consider them to be spermatogonia in the 

 testis and oogonia in the ovary. They have almost reached that degree 

 of specialization at which we might call them oocytes and spermatocytes. 

 The fact that they are still found in process of mitotic divisions ex- 

 cludes them from these latter classes of cells. They should more prop- 

 erly be termed spermatogonia and oogonia of the second order, in ac- 

 cordance with the use of these terms by Loisel in the case of birds and by 

 many authors who have written upon the subject of the sex cells of 

 invertebrates. 



These primitive sex cells found in the early stages of embryonic life 

 have, then, undergone a process of precocious development; but for 

 some reason, this process is not carried beyond a certain point. The 

 stimuli or favorable conditions that brought about the formation of 

 these secondary spermatogonia in regions outside of the sex gland, may 

 be later present only in the sex gland itself, or indeed, only in certain 

 parts of it. The influence exerted by the sex gland in bringing about 

 the development of the sex cells is beautifully illustrated by the process 

 above described by which egg follicles and spermatogonia develop in 

 large numbers in the rete tubules lying within the sex gland, while they 

 deN-elop sparingly in those parts lying within the mesonephros. In such 

 cases as we have seen, the degree of development and number of sex 

 cells forming in any given region of the rete tissue is dependent upon 

 the proximity of that region to the sex gland. It must be clearly under- 

 stood that I do not deny the possibility of an early specialization and 

 segregation of the sex cells as claimed by Nussbaum, 80, Eabl, 96, 



