176 Sex- Reversal in Frogs and Toads 



separate zygotes, so far as the evidence from sixty-one cases goes. The 

 embryonic membrane of such twins, however, fuse in an early stage 

 (embryos of about 30 mm.) and the blood-vessels of the two individuals 

 anastomose. If one is male and the other female the reproductive 

 system of the latter fails to develop its usual characters, and characters 

 of the male appear instead to a variable extent which appears to depend 

 upon variations in time and degree of the vascular anastomosis. Such 

 individuals have long been known as free-martins. The gonad is testis- 

 like in form and structure owing to complete suppression of the ovarian 

 cortex and hypertrophy of the homologue of the seminiferous tubules. 

 The Miillerian ducts usually degenerate, and the Wolffian ducts may 

 develop into quite typical vasa efferentia ; guhernacula arise as in the 

 male ; but, save in very exceptional cases, the external organs of repro- 

 duction and the mammary gland conform to the female type. In rare 

 cases (about one in eight cases of two-sexed twins) the vascular anasto- 

 mosis fails to develop, and in such cases the female is normal. No 

 abnormalities of the reproductive system of the male arise in two-sexed 

 twins. 



Sex-determination in the zygotic sense is thus seen not to be the 

 exclusive determiner of sex-differentiation in mammals, even in respect 

 to the most fundamental sex-characteristics. The possibility of complete 

 sexual inversion, by means of hormones of the opposite sex, and of 

 control of sex-determination in this sense, is thus postulated. 



Chapin who made a microscopic study of the reproductive system of 

 foetal free-martins demonstrated that there is a fusion of the embryonic 

 membranes and a subsequent anastomosis of blood vessels of the cattle 

 twins. If one twin be male and one be female, the latter is commonly 

 sterile. This is the result of the introduction of the interstitial secretion 

 of the male into the blood of the female. It is manifested by the de- 

 velopment toward the male condition, of those organs in the free-martin 

 which are present in the indifferent stage {rete, first set of sex cords, 

 primary albuginea), and the absence of those organs which develop in 

 the normal female at sex differentiation or later (cords of Pfluger, defini- 

 tive albuginea, union of Miillerian ducts to form uterus). 



There is much variation in the reproductive organs of the free- 

 martins. This is due to two variable factors : (1) the time at which the 

 interstitial secretion of the male embryo may first enter the circulation 

 of the female embryo, and (2) the amount of secretion which may be 

 introduced. 



Willier who has more recently investigated the structure and homo- 



