456 



EMBRYOLOGY OF THE CHICK 



The lips of the blastopore form a region of rapid cell proliferation, 

 though all the cells look quite alike. Nevertheless, it is from these rap- 

 idly proliferating cells that the various germ layers are derived. 



Figure 263 shows an enlarged longitudinal, as well as a cross section 

 of an early embryo. As the lips of the blastopore grow closer and 

 closer together, they finally fuse, forming the primitive streak. Ecto- 

 derm and entoderm cannot be distinguished, but from the thickened ap- 

 proximation of the lips of the blastopore there is an inward growth of 



y.f. l.b. 



Fig. 263. 



A. From medial longitudinal section through embryonic disk of Chick. 

 Bonnet. 



B. From transverse section through Hensen's node germ disk of chick of 2 

 to 6 hours' incubation. Duval. For lettering see Fig. C. 



C. From transverse section through primitive groove germ disk of chick of 2 

 to 6 hours' incubation. Duval. arc., Archenteron ; ec., ectoderm ; en., entoderm ; 

 l.b., lip of blastopore ; p.g., primitive groove ; y., yolk ; y.p., yolk plug. 



a single layer of cells now called entoderm, and from between these two 

 layers some rather loosely arranged cells form a third layer, considered 

 the primitive origin of what is later to be called mesoderm. 



At the same time this mesodermal outgrowth appears, the dipping 

 down of the outer layer occurs, forming the primitive groove. 



The three layers which have thus been established are very impor- 

 tant, because in all forms of animal life so far studied, there is a decided 

 similarity in the origins and developments of the various organ systems. 

 Therefore, an understanding of the way the germ-layers and the organ 

 systems arise, alone permits an understanding of the ever increasing 

 perplexities coming forth as these in turn develop further. 



In our study of comparative anatomy we shall see why it is that 



