40 EARLY EMBRYOLOGY OF THE CHICK 



metamerically arranged cell masses or somites they are fre- 

 quently designated as the segmental zones of the mesoderm. 

 The segmental zones are in early stages most clearly marked 

 somewhat cephalic to Hensen's node, -where the first somites 

 will appear. As they extend caudad on either side of the 

 primitive streak they gradually become less and less definite. 



The sheet-like layers of mesoderm which are characteristic 

 of the mid-body region do not extend to the anterior part of 

 the embryo. The mesoderm of the future head region is 

 derived from mesoderm cells which invade the head from the 

 more definitely organized layers of mesoderm lying posterior to 

 it. The cephalic mesoderm for this reason never shows the 

 regional differentiations and the organization into definite layers 

 which later appear in the mesoderm of the mid-body region. 



The Formation of the Notochord. The no tochord arises in the 

 chick as a median out-growth from the rapidly proliferating, 

 undifferentiated cells at the cephalic end of the primitive streak 

 (Fig. 13, F). The way in which the no tochord grows cephalad 

 from the anterior end of the primitive streak, just as in other 

 vertebrate embryos it arises from the region of the anterior lip of 

 the blastopore, is one of the points which confirms the identifica- 

 tion of the primitive streak of the chick as the closed blastopore. 



Largely because of the way in which the notochord arises in 

 Amphioxus, a primitive vertebrate of doubtful relationships, it 

 has usually been considered of entodermal origin. In Amphibia 

 and in birds it arises not from any definite germ layer but from 

 the undifferentiated growth center about the blastopore which 

 is giving rise to both entoderm and mesoderm. Even in Am- 

 phioxus the notochord arises at the same time and in the same 

 manner as the mesoderm. In its later differentiation the noto- 

 chord resembles mesodermal derivatives more closely than 

 entodermal. The common origin of notochord and mesoderm, 

 and the unmistakably mesodermal characteristics of the fully 

 developed notochord should be emphasized rather than the 

 early association of the notochordal primordium with the 

 entoderm and its doubtful origin therefrom. For these reasons 

 the notochord is in this book treated as a mesodermal structure. 



In entire embryos of 18 to 22 hours (Figs, n and 14) the 

 notochord can be seen in the mid-line extending cephalad from 

 Hensen's node. Hensen's node is at once the posterior limit 



