582 DEVELOPMENT OF CHICK CHAP. 



lively very small and the ectoderm separated from the 

 endoderm by the yolk. 



The gastrula-stage is much less clearly distinguish- 

 able in the segmenting telolecithal eggs of the dogfish 

 and bird (pp. 470 and 583), in which the relatively 

 enormous mass of unsegmented yolk. is, as in the crayfish, 

 sufficient to nourish the embryo until it has reached a 

 stage closely resembling the adult in almost every 

 essential respect except size. A blast opore can sometimes 

 be recognised in such cases, but in the embryo of 

 the common fowl it is to some extent represented by a 

 primitive streak and groove (see p. 581 and Fig. 153, E, 

 pr. st). The blastoderm becomes differentiated into an 

 outer ectoderm and an inner, lower layer of cells (com- 

 pare Fig. 128), between which and the yolk the enteric 

 cavity is formed : a segmentation-cavity is recognisable 

 in early stages. As the embryo develops, it becomes 

 folded off from the yolk, which is gradually reduced in 

 amount and in later stages is contained in an extra- 

 embryonic part of the embryo the yolk-sac (Figs. 129 

 and 159). 



A more detailed account of the early stages of the 

 chick is given below, as the fowl's egg is an easily 

 procurable type for the practical study of develop- 

 ment. 



The fowl's egg has already been described in detail (p. 565, 

 Fig. 149). Its essential part, the yolk, corresponds, as we 

 have seen, to a single cell enormously enlarged by the mass < 

 of yolk-granules it contains, the only part in which they are 

 absent being a small area which, owing to its lesser specific 

 gravity, is always found at the upper pole ; this area, in 

 which the nucleus lies, is known as the germinal disc. Fer- 

 tilisation takes place in the anterior end of the oviduct, and 

 then the nucleus divides and initiates the process of seg- 

 mentation which is completed by the time the egg is laid, 

 when the germinal disc has thus become converted into a 

 multicellular blastoderm (p, 574). 



