648 embryology: chick 



comes over the lower layer cells of this area, most of which 

 become flattened and adhere by their edges to form a continuous 

 layer (Fig. 504, hyp.), the definitive endoderm for the gut. The 

 endoderm in the area opaca is formed, from the lower layer cells 

 there, somewhat later ; its cells are cubical and have the function 

 of taking up yolk and passing it to the blood vessels which form 

 in the mesoderm above them (p. 658). A few lower layer cells 

 remain in the blastocoele and will join the mesenchyme (loose 

 mesoderm, p. 638). According to a recent description of the pro- 

 cess the endoderm of the area pellucida arises by a migration 

 inwards and forwards of cells from a region of the surface of the 

 blastoderm at what will be the hinder end of the embryo. 



While the gut endoderm is forming, the area pellucida is 

 becoming pear-shaped, with the broad end forwards (Fig. 503, 3). 



i Jl- -TTZeS. 



Fig. 505. — A section across the primitive streak of a chick embryo. 

 cp., ectoderm ; hyp., endoderm ; mes., mesoderm ; pr.gr., primitive groove. 



As this happens, there appears in the narrow end an opaque 

 strip, the primitive streak, and along this a primitive groove 

 develops. This is formed by an immigration of cells from the 

 upper layer, which spread out as a third layer, the mesoblast, 

 between epiblast and hypoblast (Fig. 505). The history of the 

 primitive streak of the frog (p. 633) shows that this streak 

 represents the blastopore. Before their immigration these cells 

 lie in ' presumptive areas ' of the surface in the same way as 

 the corresponding cells of the frog's embryo (Fig. 507). The cells 

 which will form the lateral plate mesoderm are the first to 

 immigrate ; those which form the epimeres follow ; those for the 

 notochord enter last and form, at the anterior end of the streak, 

 a swelhng, the primitive knot, which recedes (see below) as it 

 forms the notochord. 



GASTRULA 



There are now present in the bird's embryo all the parts of 

 a gastrula (p. 621) — ectoderm, endoderm, archenteron (sub- 

 germinal cavity), and blastocoele, though the layers do not form 



