FRANK R. IJLLIE. 



crus and pes are separate chondrifications. The crus and pes 

 are thus certainly not represented in the rudimentary hind-limb 

 of this embryo. 



In the normal chick the seventh postbrachial nerve innervates 

 only some of the proximal muscles of the leg, those extending 

 from the pelvis to the femur. On the principle that the muscular 

 distribution of the nerve is confined to the derivatives of the cor- 

 responding somite, it follows that the seventh postbrachial somite 

 contributes to the formation of only the upper segment of the leg. 



\ 



FIG. 3. Sagittal section of embryo shown in Fig. 2, cut considerably to the left 

 of the middle line. A, amnion ; B.P., brachial plexus (nerves of wing) ; /, poster- 

 ior end of intestine ; IV. D., Wolffian duct ; 1-7 first to seventh postbrachial nerves. 



The absence of the crus and pes is therefore what might be 

 expected if the principle of self-differentiation were rigidly adhered 

 to. We have seen that, as a matter of fact, these are absent in 

 the rudimentary hind limbs of this embryo, so we may conclude, 

 at least, that there is no evidence that the seventh postbrachial 

 somite has produced anything beyond the normal as a result of 

 the absence of the following somites of the leg. 



Alimentary Tract. Practically all of the splanchnopleure of 

 the embryo posterior to the vitelline arteries was destroyed by 



