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 

 briefl}^ referred to, which is known as segmentation (Fig. 8). 

 This either affects the entire substance (holohlastic or complete 





Fig. S. — Various stages in the segmentation of the ovum. (From Gegeubaur's Comparative 



Anatomy.) 



segmentation) or only a part {meroUastic 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 si.xteen, 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 

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



, arch' 



A B 



Fio. 9.— Gastrulatlon. 



arch, archenteron ; bl. 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 hod,// or morula 

 stage. The blastomeres next become arranged regularly in a 

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



