42 



THE DEVELOPMENT OF THE CHICK 



well as deep boundaries; they may be called the marginal cells 

 (Fig. 16 D). The distinction between central and marginal 

 cells is one of great importance which should be clearly 

 grasped. 



In the surface views of later cleavages the following points 

 should be noted: (1) the group of central cells increases by the 

 addition of cells cut off from the inner ends of the marginal cells, 

 and by the multiplication of the central cells themselves; (2) the 

 marginal cells increase by the formation of new radial furrows. 

 The increase of the central cells is much more rapid than that of 



Fig. 16 A. Median section of a blastoderm of the hen's egg which showed 

 about 64 cells in surface view (after Patterson). S.c, segmentation 

 cavity. 



the marginal cells, and the cells themselves are much smaller than 

 the marginal cells, both because of their mode of origin and also 

 because of their more rapid multiplication. The area of the 

 central cells is also constantly increasing, with consequent re- 

 duction of the marginal zone (Fig. 16 E). Emphasis has been 

 laid by several authors on the excentric position of the smallest 

 cells, and the inference has been drawn that these represent the 

 hinder end of the glastodisc. Similar excentricity in the pigeon's 

 egg is without reference to the future embryonic axis (see 

 Fig. 18). 



But the surface views do not show what is going on in the 

 deeper parts of the germinal disc. At the eight-celled stage a 

 narrow space appears in the depth of the central portion of the 

 blastoderm approximately between protoplasm and yolk; this is 



