ACTION OF SEX HORMONES IN FOETAL LIFE 405 



gators. While it wdll no doubt be desirable to clear up these 

 assumptions by farther observations and experiments, it is never- 

 theless true that the facts that we have presented form a solid 

 basis for both assumptions, which are in no sense unsupported or 

 forced. 



The material presents the problem whether the intermixture of 

 other internal secretions o the twin individuals modifies any of 

 their characters? It is obvious that modifications could be ex- 

 pected only in the case of differentiating characters, of which the 

 most fundamental are those of sex; all other main features of 

 organization are common to the sexes, and there is no evidence 

 of individual or sexual differences in the hormones of a species 

 except in the sexual hormones themselves. A negative answer 

 would therefore be expected to this question in general. Indi- 

 vidual variations of course exist, and it is an interesting question 

 whether the blood community of foetal life tends to reduce such 

 variations and to approximate their resemblances towards the 

 correlation of variability of identical twins? We have seen 

 reason to believe that this is not the case in regard to color, but 

 the problem remains for future investigation. 



According to the conceptions involved in this discussion, the 

 de^'iations of the sterile free-martin from the female type are 

 due to the action of the male sex-hormones. In order to appre- 

 ciate the full extent of this action, it is necessary to understand 

 minutely the anatomy of the sterile free-martin in comparison 

 with normal individuals. This comparison is made in a de- 

 tailed way for numerous cases in section V. The results there 

 given will enable us to correlate the whole series of phenomena 

 and to discuss in a very general way the phenomena of sex-differ- 

 entiation in mammals. 



In terminating the present section then we may repeat .our 

 main steps up to this point: — (1) We have demonstrated the 

 separate zygotic origin of the free-martin and its male twin, 

 (2) We have studied the foetal vascular connections in twin 

 pregnancies of cattle. (3) We have explained the existence of 

 occasional fertile free-martins. (4) We have explained the reason 

 for the usual Hmitation of sterility of the female of two-sexed 



THU JOURNAL OF EXPEHIMENTAL ZOOLOGY, VOL. 23, NO. 2 



