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. 16C). 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 blastodise. 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 
dise, 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 dise 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 dise 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. 
