ACTION OF SEX HORMONES IN FOETAL LIFE 401 



Fernandez ('15) has described fusion of dizygotic chorions in 

 one of the armadilloes (Dasypus villosus). This form has usually 

 two young at a birth, which may be one-sexed or two-sexed. 

 The two chorionic vesicles are separate at first, but they gradu- 

 ally fuse and by the time that the embryos are 3.5 cm. long the 

 fusion of the two vesicles is so intimate that they appear as a 

 single one. As this is the usual condition it cannot be supposed 

 that it is accompanied by genital abnormalities. The author 

 does not consider the problem of vascular anastomosis; but it 

 can hardly be supposed to occur. 



Although the hormone theory invokes a cause of the utmost 

 generality in mammals, it is obvious that the conditions leading 

 to its intervention must be restricted to forms with a relatively 

 diffuse placentation, i.e., ungulates for the most part, and among 

 these to forms in which quite special conditions obtain. Such 

 conditions are found only in normally uniparous ungulates in 

 which the ovum grows to an extreme length very rapidly, so 

 that the associated ova meet at an early stage which favors their 

 organic union. Even then vascular anastomosis is not likely 

 to occur to any considerable extent unless the development of 

 the foetal cotyledons is relatively late, so as to be preceded by a 

 condition of general vascularization of the chorion, before the 

 highly specialized circulation of the cotyledons becomes domi- 

 nant. Such is the condition in cattle. In sheep the develop- 

 ment of the cotyledons appears to be more precocious; I would 

 at least venture this suggestion, although based on relatively 

 few observations, to explain the difference between cattle and 

 sheep in this respect. 



Although these considerations give a color of great probability 

 to the hormone theory we have still to deal with two difficulties : — 

 1) it is evident that we must be dealing with specific sex hor- 

 mones, for their influence is limited to sex characters so far as 

 our present evidence goes; is there any other evidence for such 

 early production of specific sex hormones available? 2) It is 

 certain that the intermixture of blood of the twins must be re- 

 ciprocal, but the effect is exclusively on the female; in what way 

 can this be explained? 



