PRIMITIVE STREAK TO SOMITE FORMATION 41 



of the notochord and the anterior end of the primitive streak. 

 The notochord and the primitive streak together clearly mark 

 the mid-line of the embryo and establish definitely the longitu- 

 dinal axis of the developing body. In sections (Fig. 13, A, F) 

 the notochord is not at this early stage sharply differentiated 

 from the loosely arranged mesoderm cells adjacent to it. In 

 later stages, however, the cells composing it become aggregated 

 to form a characteristic rod-shaped structure, circular in cross 

 section and with clearly defined boundaries (Fig. 52, C). 



The Formation of the Neural Plate. In surface views of en- 

 tire chicks of about 18 hours (Fig. n) areas of greater density 



cephalic end 



proamnion 



subcephalic 

 pocket 



ectoderm of head 



area pellucida 



border of fore-gut 



margin of anterior 

 intestinal portal 



notochord 



primitive streak 



area opaca 



caudal end 

 FIG. 14. Dorsal view ( X 14) of entire chick embryo of about 21 hours incubation. 



may be made out on either side of the notochord. These areas 

 extend somewhat anterior to the cephalic end of the notochord 

 where they appear to blend with each other in the mid-line. 

 Sections of this region (Fig. 13, A) show that the greater 

 density seen in whole-mounts is due to thickening of the ecto- 

 derm. Rapid cell proliferation has resulted in the ectoderm 

 in the middle region becoming several cells in thickness. This 



