22 



ZOOLOGY 



SECT. 



in the polar bodies and also minus its centrosome, and plus 

 the sperm with its nucleus and centrosome. 



On impregnation follows shortly the process of division already 

 briefly referred to, which is known as segmentation (Fig. 8). 

 This either affects the entire substance (holoblastic or complete 



FIG. 8. Various stages in the segmentation of the ovum. (From Gegenbaur's Comparative 



Anatomy.) 



segmentation) or only a part (mcroUastic or incomplete seg- 

 mentation) of the oosperm. In the former case the ovum usually 

 contains little or no food-yolk, consisting exclusively, or nearly 

 so, of protoplasmic matter. The first stage in the process of 

 segmentation is the mitotic division of the segmentation-nucleus, 

 accompanied by the division into two parts of the substance 

 of the protoplasm the result being the formation of two cells, 

 each with its nucleus (Fig. 8). Each of these two cells then divides 

 four cells being thus formed ; the four divide to form eight ; 

 the eight divide to form sixteen, and so on ; until, by the process 

 of division and subdivision, the oosperm becomes segmented into 

 a large number of comparatively small cells which are termed the 

 Uastomeres. This mass of cells is spherical in shape, and the 



ABC 



FIG. 9. Gastrulation. 



arch, archenteron.; W. blastopore ; ecto. ectoderm ; endo. endoderm. 



rounded blastomeres of which it is composed project on its sur- 

 face so as to give it somewhat the appearance of the fruit of 

 the mulberry, whence it is termed the mulberry body or morula 

 stage. The blastomeres next become arranged regularly in a 

 single layer the embryo (Fig. 9, A) assuming the form of a hollow 



