458 



EMBRYOLOGY OF THE CHICK 



it in the midline except posterior to the primitive streak; but, immedi- 

 ately on each side of the midline, the mesoderm is quite thickened, thin- 

 ning out as it extends toward each side. The dorsal mesodermic plates 

 are to develop from these thickened portions of the mesoderm, and as 

 they will then segment, they are called segmental zones of mesoderm. 

 The first somites will appear cephalad to Hensen's node, extending can- 

 daily along each side of the primitive streak and becoming less and less 

 distinct. 



Fig. 265. 



From medial vertical sections through embryonic disk of lizard, 

 showing five successive stages in gastrulation (Wenckeback, 

 Bonnet). 



It is important -to note here that the sheet-like layers of mesoderm 

 so characteristic in the mid-body region do not extend to the head region 

 of the embryo. The mesoderm of the head region develops from quite 

 definitely organized layers immediately behind the future head. The 

 reason that the mesoderm of the head is separate in origin from that of 

 the remaining portion of the body may be accounted for by the fact that 

 the head is not segmented as is the mesoderm of the body-region. 



THE NOTOCHORD 



From the cephalic end of the primitive streak the rapidly prolif- 

 erating cells extend in an anterior manner. In non-bird-like vertebrates, 



