402 FRANK R. LILLIE 



Both of these difficulties receive a satisfactory explanation in 

 certain previously known facts not hitherto correlated with the 

 phenomenona in question, and in certain new facts which are 

 described by Miss Chapin ('17). They constitute concurrent 

 evidence which appears to me to render the entire evidence per- 

 fectly conclusive. 



The previously known facts to which I refer are (1) the early 

 development of the interstitial tissue of the mammalian testes, 

 from the very beginning of sex-differentiation, and (2) the fact 

 that the differentiation of the ovary is later than that of the 

 testis, inasmuch as in the female of manunals the first generation 

 of ingrowths from the germinal epithelium, a complete homo- 

 logue of the seminiferous tubules of the male, forms only the 

 medulla of the ovary, and the ovarian cortex is formed from a 

 distinct second generation of ingrowths. Thus (1) interstitial 

 tissue of the testis is present at the time for which male hormones 

 are postulated, and (2) the testis has a start over the ovary in 

 this respect which results in the suppression of specific ovarian 

 tissue from the beginning as shown by Miss Chapin' s study. 



On the question of the embryonic origin of interstitial cells in 

 the testes we have the excellent studies of Whitehead ('04) and 

 Allen ('04) on the pig; Allen also deals to a certain extent with 

 the interstitial cells of the ovary. Allen finds that sex differen- 

 tiation is strikingly shown in the structure of the testis and 

 ovary in embryos of 2.5 cm. length. The sexes cannot be sharply 

 distinguished at 1.8 cm. length so that the initial stages of sex- 

 differentiation lie in between. Interstitial cells are present in 

 both testis and ovary at the stage of 2.5 cm., but while they are 

 very numerous in the testis at this stage they are very rare in 

 the ovary. Whitehead finds that they appear in the testis of the 

 pig embryo at 2.4 cm. Althoughr these authors do not note it, 

 it is unquestionably significant that these cells appear, and ex- 

 hibit the usual evidences of active secretion at the time of the 

 onset of sex-differentiation. It is also significant that they appear 

 first in the testis and more abundantly than in the ovary; this is 

 of course correlated with the fact that the cortex of the ovary 

 in which they appear is a later formation than the seminiferous 

 tubules or their homologue in the female, the medullary cords. 



