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PROCEEDINGS OF THE ANATOMICAL AND ANTHROPOLOGICAL 



attempted must suffice for both cases. A further consideration of 

 the alimentary variations shows that they are to l>e referred to that 

 portion of the embryonic gut termed the mid-gut, and in my opinion 

 these variations are entirely referable to the mid-gut, though this, as 

 will be presently shown, is open to question ; but putting the debatable 

 point on one side, that is the origin of the liver whether from fore-gut 

 or mid-gut, it is certain that the jejunum is a mid-gut development, 

 ami as both cases have only one jejunum it is safe to assume that the 

 two embryos in each case have had only one yolk-sac between them, 

 that is to say, they have been developed from one ovum. 



Fig. 2. Diagram to illustrate the body fusion. 



Commencing upon this hypothesis, and remembering the lateral 

 nature of the fusion of the twins, I first assumed that two primitive 

 grooves had been lying side by side upon a single ovum. I en- 

 deavoured to account for these two primitive grooves by assuming 

 bilateral segmentation of the ovum, but I was forced to abandon this 

 position as untenable inasmuch as if worked out it will be found to 

 necessitate the formation of two yolk-sacs and two amnions, and their 

 subsequent conversion into single structures by fusion. Quite apart 



