SEX-INTERGRADES IN POSTAL PIGS. 123 



an unequal sharing of the blood supply, one twin receiving an 

 excess of blood and thereby robbing its co-twin of the full amount 

 required for normal development. In one instance, the twins 

 measured 30 mm. and 13 mm. respectively. The smaller 

 individual had not commenced disintegration, but was a case 

 of acardia with several morphological abnormalities. 



In 4 cases, the establishment of vascular inter-communications 

 was followed by abnormalities of the sex-equipment of one of 

 the component twins. I now propose to describe these four 

 cases. 



Case No. I. (Figs, i and 3.) The twins in this case measured 

 65 mm. and were heterosexual as determined by the external 

 genitalia. The normal length of the chorionic vesicle, and the 

 position of the embryos in its center, indicate that fusion had 

 taken place in a side-by-side manner. Fig. I is a semi-diagram- 

 matic representation of the chorionic vesicle and its contents. 

 In the foreground is the amnion enclosing the female embryo 

 (A), behind which its allantois extends the complete length of 

 the exoccelom. The constriction in the allantois to the right 

 of the embryo is no doubt caused by pressure of the male embryo 

 (B), which lies behind allantois A at this point. The allantois 

 of B lies in the background, and also extends the complete 

 length of the vesicle. 



The distribution of the allantoic vessels is irregular. The 

 greater portion of the placenta is vascularized by embryo A, 

 whose anterior and posterior paired vessels are of equal strength 

 and course the total length of the chorion. Those of B are 

 unequal, the anterior pair being extremely weak and lost to 

 view upon entering the placenta. The stronger posterior pair 

 courses side by side with the corresponding pair from A , and in 

 more than one place the branches of the two systems anastomose. 



On dissection of the embryos, the reduced size of the female 

 gonads was at once apparent. Actual measurements were 2.2 

 mm. in length compared with 4.4 mm. of the testes. A difference 

 as great as this I have not observed in any normal individuals 

 at the same stage of development. Histological examination 

 showed structural modifications similar in most respects to those 

 described for early free-martins. Figs. 5, 6 and 7 are camera 



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