110 BENJAMIN H. WILLIER 



Just how much growth and differentiation of the free-martin 

 gonad occur after birth has not been determined, owing to the 

 difficulty of obtaining the late foetal stages. In the early 

 free-martin embryos examined (oldest embryo was 28 cm. long), 

 the gonad is smaller than normal and the seminal vesicles have 

 not yet appeared. In the normal male the seminal vesicles are 

 well indicated from 16.8 cm. on. In approximately 80 per cent 

 of the post-natal free-martins the seminal vesicles are well- 

 defined structures. In some the prostate appears (H-18) (cf. 

 literature). It would appear, therefore, that there is a delay 

 in growth during the late prenatal stages. With this gap filled, 

 it would be possible to make a more careful analysis of the factors 

 which are responsible for the later steps in the modification of 

 the reproductive system. At birth the possibility of any further 

 action of the male sex hormones terminates. During this period 

 of late foetal development, search should be made for any evi- 

 dence of interstitial cell activity in the testis of the male twin, 

 and also to determine the degree of anastomosis of the extra- 

 embryonic blood-vessels between the male and female individuals. 



A parallelism is seen between the degree of transformation of 

 the gonad and the degree of transformation of other parts of 

 the internal reproductive system, the external genitaha and the 

 phenomenon of descent. In the case of the reproductive-duct 

 system the parallehsm is not distinct, but in general there is a 

 tendency for the female parts to disappear and for the male 

 parts to develop as the gonads are transformed in the male 

 direction. The external genitalia are the least liable to undergo 

 modification, but in cases where the normal male limit is closely 

 approached, they may also be transformed. That is, in cases 

 where the reproductive glands are structurally typical testes, 

 and have descended into the groin region, the external genitaha 

 assume male characters (the clitoris in 11-37 is partially trans- 

 formed into a penis). Obviously, the external genitaha, the 

 system of ducts and accessory reproductive glands as well as 

 the gonads exhibit a graded series of transformations in the male 

 direction. 



