IV 



FORMATION AND COMPLETION OF THE BLASTODERM 



In the preceding section the cleavage cells were followed to 

 the point where they reach the periphery. This stage in their 

 migration is illustrated by figures 7 A and nA. In the second 

 figure mentioned only a single cell is represented, much magni- 

 fied, together with a corresponding portion of the cortical layer. 

 A distinction is now apparent between the protoplasm of the 

 cortical layer and that of the cleavage cells. The former is 

 clear and transparent, the latter dark and granular. The cleav- 

 age cells rest upon the cortical layer as upon a base, supported 

 by short intervening processes which are separated from one 

 another by irregularly circular openings. On meeting the corti- 

 cal layer these processes spread out to unite under this layer to 

 form a continuous sheet. On comparing figure /A with an ear- 

 lier stage, figure 6, it becomes evident that these processes ex- 

 tending peripherad from the cleavage cells are only the strands 

 of the protoplasmic meshwork underlying the cortical layer, 

 shortened and thickened, while the openings represent the spaces 

 occupied by the vitelline spheres. As the cells advance peri- 

 pherad the processes shorten, while the open spaces flatten and 

 ultimately disappear (Fig. nB) ; the nucleus of each cell is thus 

 brought into contact with the cortical layer (Fig. nC). The 

 nucleus continues to advance peripherad, followed by the deep 

 staining protoplasm of the cell body, while over it the cortical 

 layer begins to rise up on the exterior in a more or less rounded 

 swelling, each of these being marked off from its neighbors by a 

 wide and deep furrow (Figs. nC, D and E). This stage, when 

 seen in the living egg, presents a very striking and characteristic 

 appearance, and has been well described and figured by Kowa- 

 levski (1871). A differentiation in the character of the proto- 

 plasm of the cell body now begins to appear. That part 

 accompanying the nucleus is seen to be less deeply stained and 



27 



