208 EMBRYOLOGY 



endoderm. Similarly, the notochord cells will remain separate from either the 

 mesoderm or the endoderm. 



To continue with our comparison of gastrulation in chick and amphibian, 

 the head process, which differentiates into notochord, extends forward from 

 the anterior end of the primitive streak (Fig. 121) . In the amphibian gastrula 

 the notochord turns in at the dorsal lip of the blastopore and extends 

 anteriorly. If we consider the anterior end of the primitive streak as homolo- 

 gous with the dorsal lip of the blastopore, we may say that the notochord 

 forms in the same way in both chick and amphibian. 



Finally, the proliferation of the mesoderm from the sides of the primitive 

 streak (Fig. 122) resembles the migration of the mesoderm over the lateral 

 lips of the amphibian blastopore and its subsequent spreading between the 

 ectoderm and endoderm. 



Development during the first day 

 of incubation 



The age of chick embryos is designated in terms of hours of incubation 

 and in terms of number of somites, as discussed in Chapter 12. During the 

 first 24 hours of incubation five pairs of somites form. 



Inspection of a stained preparation of the primitive-streak blastoderm 

 (Fig. 123) shows an over-all division into an area opaca, which is in contact 

 with underlying yolk, and an area pellucida, in which the cells are separated 

 from the yolk and appear more translucent. The primitive streak appears 

 as a denser region in the long axis of the ovoid area pellucida. The mid- 

 region of the primitive streak appears lighter, owing to a groove — the 

 primitive groove — which runs along the length of the streak. The posterior 

 end of the streak is closer to the area opaca than the anterior end. At the 

 anterior end a dark mass of cells forms the primitive knot (Hensen's node), 

 and these cells will give rise to the notochord. Surrounding the anterior end 

 of the streak lie the ectoderm cells which will form the forebrain and eye. 

 The ectoderm on each side of the streak for about half its length will differ- 

 entiate into midbrain, hindbrain, and spinal cord. Much of the mesoderm 

 has proliferated from the sides of the primitive streak and forms a layer 

 between the ectoderm and the endoderm, as in Figure 122. 



In the area opaca the mesoderm cells have aggregated into small clumps, 



