Development of Chick 



43 



the direction of the future main axis, is thicker towards its posterior 

 end. 



Of course after laying there is an almost complete cessation of 

 the developmental processes as the egg cools, so the egg remains 

 in a dormant state, though in hot summer weather development 

 makes some slight progress. Then as soon as incubation begins 

 the cells again become active and the changes effected are rapid. 

 During the first few hours the chief change is in connection with 

 the margin of the blastoderm. 



The groove seen in the unincubated egg quite disappears and 

 the outermost layer of cells extends over the yolk beyond the 

 zone where nuclei lie scattered within the yolk. This layer is now 

 quite distinct from the underlying cells throughout the whole area 



Fig. 24 after Duval. Section through blastoderm of chick 



incubated ten hours. 



A , anterior ; P, posterior, ect ectoderm ; end endoderm ; mes mesoderm ; 

 sg segmentation cavity; y yolk (subgerminal). 



and it constitutes the ectoderm. Its progress in the future over 

 the yolk is by its own interstitial growth. It receives no additional 

 cells or nuclei from the yolk. 



The loose cells below have by the 10th hour of incubation, 

 Fig. 24, formed a continuous sheet of tissue, this is the endoderm 

 and it is connected all round its margin with the yolk, which there 

 contains many nuclei, and it is called the germinal wall. According 

 to some authors, Duval, Marshall and Balfour, some of the cells 

 of this loose mass are left between the sheet of endoderm and the 

 ectoderm, and they are said to be more numerous in the anterior 

 part of the area pellucida. On the other hand other authors 

 (e.g. Lillie) deny the presence of these cells. 



There can be no doubt about their presence in the Sparrow, 

 where they form a many layered mass circular in outline in the 



