390 FRANK R. LILLIE 



collector did not recognize the case as a twin pregnancy until 

 after the uterus had been opened and one foetus removed, thus 

 cutting the membranes; the other foetus was also removed in 

 the same way, and when the uterus and specimens reached me 

 for examination it was necessary to reconstruct the original 

 condition from the parts. This w^as, however, successfully 

 done, as the central end of the one chorion was found still in 

 place in the body of the uterus and extending into both horns. 

 In this one the chorion "wdth contained allantois had passed the 

 body of the uterus. In the other the end of the chorion had been 

 drawn out of the uterus with the foetus, but measurement showed 

 that it also with the allantois contained had passed the body of 

 the uterus. The two chorions were thus not fused at this stage, 

 but they overlapped and were in closest apposition in the body 

 of the uterus. The conditions precedent to fusion were thus 

 fully established at this early stage long before sexual differen- 

 tiation begins. 



Comparison of these twins with the stages of 19 and 21 mm. 

 described above indicates some variation in the degree of devel- 

 opment of the chorion relative to the length of the embryo. 

 But these stages demonstrate the possibility of fusion of twin 

 chorions a considerable time before the stage of beginning sex- 

 differentiation, which I estimate at about 25 mm. Vascular 

 anastomosis between the twins is possible as soon as the allantoes 

 from the two sides meet, or even earlier, because after the allan- 

 tois has once fused to the chorion the blood-vessels tend to 

 spread out more or less in the chorion beyond the area of fusion. 



Owing to the extreme difficulty of obtaining early stages of 

 twin pregnancies in cows the next earliest stage that we have 

 (no. 51) is a case of male twins 5 cm. long in which there is no 

 evidence of the place of fusion of the twin chorions, and there 

 is a perfect vascular anastomosis between the two sides. Fusion 

 is already perfect and any overlapping parts have entirely dis- 

 appeared. The next case is a two-sexed pair (no. 19) in which the 

 male foetus was 80 mm. and the female 75 mm. long. The twin 

 chorion was single with a broad connection provided with 

 cotyledons between the two halves; no evidence of the place of 



