DEVELOPMENT OF THE CHICK 645 



of water, without which the embryo could not develop on dry 

 land. Thus birds and reptiles are freed from that dependence 

 upon a watery medium during their early stages which exists 

 for the frog. The birds, indeed, have over-compensated for the 



Fig. 501. — Views of the young blastoderm of the chick in three successive 



stages of cleavage. 

 A, Two-cell stage. B, Four-cell stage. C, Eight-cell stage. 



dryness of their environment, and their eggs actually lose water 

 during incubation ; if the atmosphere is too damp for this to 

 happen they fail to develop. Cleavage (Fig. 501) begins with the 



v.m. 





xx.jn. 



I J, 



s.j.c. 



v.rtr. 



S q c 



Fig. 502. — Sections through the young blastoderm of the chick in three successive 

 stages, of which the first {A) is that of Fig. 501, C and the second {B) a little 

 later than that of Fig. 503, i. SHghtly diagrammatic. 



bl., blastomeres ; s.g.c, subgerminal cavity ; v.m., vitelline membrane ; y., yolk. 



formation across the germinal disc of a furrow which does not 

 reach to the disc's edge. This is soon crossed by another furrow, 

 and then more appear till the disc is divided into a mosaic of 

 small irregular segments (Fig. 501, C). Sections of the disc show 

 that at the same time a horizontal cleft is forming by which the 

 segments become separated from the underlying yolk (Fig. 502, B). 



