ACTION OF SEX HORMONES IN FOETAL LIFE 403 



As regards the origin of the interstitial cells of the ovary we 

 have relatively few observations. Allen notes, as we have seen, 

 that they are very rare in the ovary of the 2.5 cm. pig as com- 

 pared with the testis; they are also very short-lived, disappear- 

 ing at the stage of 4 cm. ''No interstitial cells are found in the 

 rabbit ovary until the stage of 45 days after birth." They would 

 thus appear to form a very inconspicuous and transitory feature 

 of the embryonic ovary.' It is not clear from Allen's account 

 whether they occur in the medulla or cortex of the embryonic 

 ovary; a point which is of some significance, in connection with 

 the following discussion. 



As regards the testis \^Tiitehead notes 



Leydig's cells (the interstitial cells of the testes) pass through two 

 phases of growth, between which a phase of atrophy intervenes. 

 Growth is very rapid from their appearance in the embryo 2.4 cm. long 

 until the length of 3.5 cm. is reached. Tliis is followed by the phase 

 of atrophy, during which the cells return almost to their first state 

 of nearly naked nuclei (figs. 4 and 5). This process reaches its acme 

 in the embryo 14 cm. long. Synchronous with it there is extensive 

 growth of the seminal tubules, particularly in length, so that they are 

 much convoluted, and the intertubular spaces are correspondingly 

 narrowed (fig. 6). 



The atrophy proceeds slowly from 3.5 to 14 cm., and is not 

 at all marked at 5.5 cm. 



In the embryo 20 cm. long the cells enter upon the second phase of 

 growth, which attains its maximum in the pig of 28 cm., very near to 

 term. Here the cells are enormously increased in number and size, so 

 that they constitute the predominating feature of the microscopic 

 picture. 



It seems to me that these facts are of great significance, viz: 

 the intensive growth of the interstitial cells of the testis at the 

 time of most rapid sex differentiation, between 2.4 and 3.5 cm., 

 the subsidence of their activity after embryonic sex differentia- 

 tion is once attained, and the second phase of activity which 

 probably leads on to the juvenile sex differentiation. It would 

 thus appear that both phases of sex differentiation are covered 

 by periods of intensive acti\dty on the part of the interstitial 

 cells of the testis. 



