DEVELOPMENT OF THE CHICK 115 



is along a plane at right angles to the first. 

 As the result of repeated synchronous division, 

 it comes to pass that the germinal disc is 

 composed successively of 2, 4, 8, 16, 32, etc., 

 cells. A point is ultimately reached when 

 division ceases to be so regular and multiplica- 

 tion of cells in geometrical progression ceases 

 to obtain. 



At first the cells are not completely separated 

 from each other because the earlier processes 

 of cleavage affect only the surface of the 

 germinal disc. In other words, up to the 

 16- or 32-cell stage segmentation is effected 

 by the multiplication of nuclei separated from 

 each other by furrows which indent the surface 

 of the germinal disc, but are not continued on 

 the deeper surface of the cells. After the 

 16- or 32-cell stage the more central cells begin 

 to divide in such a fashion that new cells are 

 produced underneath them. That is to say, 

 the cleavage, which at first affected only the 

 surface of the germinal disc, begins to invade 

 the deeper parts. In this way the central 

 portion of the disc becomes changed into a 

 cellular mass separated from the underlying 

 white yolk by a small, fluid- containing space 

 known as the segmentation or subgerminal 



