6O BENNET M. ALLEN. 



This process soon ceases in the ovary, while in the testis, on the 

 other hand, division figures are found in the interstitial cells at a 

 stage as late as the 7.5 cm. embryo. In the testis of the 15 cm. 

 embryo, they have begun 1 to degenerate. This process manifests 

 itself in a shrinkage of the cytoplasm. Interstitial cells first form 

 in the testis of the rabbit embryo of a stage between seventeen 

 and twenty-one days. They are found to be still dividing by 

 mitosis eight days after birth. They are very rare, however, in 

 the stage of twenty-four days after birth. 



No interstitial cells were found in the ovary of the embryo 

 rabbit, they being first met with in females killed forty-five days 

 after birth. Here they are scarce, but unmistakable. Consider- 

 able light is thrown upon their origin by a study of the eighty-five- 

 day rabbit. In the ovary of this stage they are very common, 

 their origin from the cells of the theca interna of atretic follicles 

 being clearly shown. This, taken in connection with the addi- 

 tional fact that they make their appearance in the 2.5 cm. pig 

 embryo coincident with the fatty degeneration of the germinative 

 cells of the seminiferous tubules and their ovarian homologues, 

 together with that of many cells of the germinal epithelium, 

 would lead us to conclude that cell degeneration offers the stimu- 

 lus or condition that brings about the formation of the interstitial 

 cells. 



Interstitial cells do not develop from unmodified connective 

 tissue cells, such as those comprising the theca externa and the 

 general ovarian stroma. Such stroma cells must be transformed 

 into cells of the theca- interna by the direct or indirect influence of 

 the growing follicles, before they are again susceptible to the influ- 

 ences exerted by the process of cell degeneration. Atresia of 

 small follicles that are not surrounded by a theca-interna does not 

 bring about the formation of interstitial cells. Many such small 

 follicles are found to degenerate early and late in the history of 

 the ovary. 



No evidence has been found favoring the theory of the early 

 segregation of the sex cells, but I am not prepared to say that 

 my work has in any way tended to disprove such a theory. Sex 

 cells appear in the very youngest stages studied (pig embryo, 

 0.6 cm. length and rabbit embryo of 13 days' age). They 



