SEGMENTATION 647 



the egg, a cap of cells known as the blastoderm is formed. Between 

 it and the yolk is a space known as the subgerminal cavity. The 

 cells at the surface now become fitted regularly together and form 

 an upper layer, one cell deep, separated by a chink, the blastocoele, 

 from the rest, which are rounded, and form an irregular stratum, 

 the lower layer (Fig. 504, D). The upper layer will give rise to the 

 ectoderm (epiblast) and to the primitive streak ; the lower layer 

 is the endoderm (hypoblast) and will give rise to the lining 

 epithelium of the gut and of the various organs which will form 

 as outgrowths from that (p. 638). The subgerminal cavity will 

 become the hollow of the gut. Around the edge the lower layer 

 cells are more closely packed, and rest on the yolk, which in 



6ic. 



\ , bic" ^hyp. 



rrvch. 



Fig. 504. — Sections through the blastoderm of the chick in two stages (D, E) 

 which succeed that of Fig. 502, C, slightly diagrammatic. 



blc, blastocoele ; ep., ectcxierm ; hyp., endoderm ; /./., lower layer cells ; mch., cells which will join the 



mesenchyme. 



this region contains nuclei derived from them and is known as 

 the germinal wall (Fig. 503, 2, g.w.). At this stage the egg is laid, 

 and the low temperature outside the body of its mother brings 

 its development to a standstill. When, however, the mother 

 begins to sit, or the egg is put into an incubator, so that the 

 temperature rises, changes take place rapidly. The blastoderm 

 spreads over the yolk, chiefly owing to the addition to its margin 

 of cells formed around nuclei of the germinal wall, which recedes, 

 by growing on the outside while it forms blastoderm on the inside. 

 In a surface view of the blastoderm there is now a dark, trans- 

 lucent area pellucida (light in Fig. 503, 3) over the subgerminal 

 cavity, and a whiter area opaca where the blastoderm rests on 

 the yolk. It is in the area pellucida that the body of the chick will 

 be formed. As the blastoderm continues to spread, a change 



