The Development of the Chick 103 



cells by these vertical and horizontal cleavage 

 planes. The cells nearer the centre of the 

 germinal disc continue to be smaller than 

 those nearer the periphery until nearly the 

 close of segmentation (Fig 35), when the 

 peripheral cells continue to divide for a little 

 longer time and thus are finally reduced to 

 the same size as the central cells. Each of 

 these small cells contains a nucleus which is 

 probably derived from the repeated division 

 of the original segmentation nucleus of the 



THE BLASTODERM 



By the time the egg is laid, the germinal 

 disc has been changed, by this process of 

 segmentation, to a sharply defined circular 

 cap of cells which, by its less specific gravity, 

 always lies at the upper pole of the egg, as 

 has already been mentioned (Fig. 33 bl). 

 This circular cap or blastoderm is usually 

 about 3.5 mm. in diameter when the egg is 

 laid, though its condition at that time varies 

 somewhat, depending upon the length of time 

 the egg remains in the uterus before being 

 laid. 



If examined carefully from the surface, the 



