42 THE DEVELOPMENT OF THE CHICK 



the central cells. These are bounded by cells that are united 

 in the marginal periblast, and thus lack marginal boundaries as 

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

 (Fig. 16 C). 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 

 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 blastodisc. 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. Sections show that after 

 about the 16- or 32-celled stage an entirely new class of cleav- 

 age planes arises in the central cells. These planes are parallel to 

 the surface, and the superficial cells arising from such a division 

 are therefore completed below. Of the two daughter-nuclei 

 produced by such a division, one remains in the superficial cell 

 and the other in the unsegmented deep layer of the germinal 

 disc, which thus becomes nucleated. After this the nuclei mul- 

 tiply in this deeper layer and cells are constantly being produced, 

 which bud off from it and become added to the segmented part 

 of the germinal disc above. 



In this way the entire thickness of the central part of the 

 germinal disc becomes gradually converted into cells. A cavity 

 arises between the cellular disc and the white yolk below, the seg- 

 mentation cavity, often called the subgerminal cavity. It is first 

 formed in the center of the central group of cells and extends out 

 gradually towards the margin, but it never cuts under the mar- 

 ginal cells, which remain united below and at their margins by the 

 periblast. 



