DEVELOPMENT OF EMBRYO OF FOWL. 



35 



ing a considerable number of potential somites. As will be 

 shown later, somites 26 to 32 are the ones that normally form 

 the musculature of the leg. Thus the undivided mesoblast 

 at this stage includes a large part of the trunk. The vitelline 

 arteries at this time leave the embryo opposite to the twenty- 

 first or twenty-second somites ; this position appears to be 

 very constant. Thus the theoretical limit of the experiments is 

 from the hind end to about the twenty-second somite, because 



-18 



FIG. i. Camera drawing of chick embryo with 29 somites; operation diagram. 



For explanation see text. 



the embryo cannot live if the vitelline arteries are destroyed. The 

 rudiment of the allantois lies beneath the unsegmented mesoblast 

 beginning, approximately, a short distance back of the thirtieth 

 somite. 



Fig. i serves at the same time as a diagram of the operations. 

 At the time of each operation a sketch was made of the embryo, 

 and the part destroyed was indicated by shading the posterior end 

 correspondingly. This sketch, naturally, did not show somites, 

 so the present operation diagram is constructed from data deter- 



