20 ZOOLOGY 



sperm coalesce with the two formed by division of that belonging 

 to the ovum ; but, more commonly, the centrosome of the ovum 

 disappears before the two nuclei come into contact. The result of 

 these changes is the formation of the impregnated ovum, or oosperm 

 as it is called. The oosperm, it is to be noted, before development 

 begins, consists of the primary ovum minus the portions of the 

 substance of its nucleus removed in the polar bodies, and also, 

 usually, minus its centrosome, sad 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. 6). 

 This "either affects the entire substance (holoblastic or complete 

 segmentation) or only a part (merollastic 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 



FIG. 0. Various stages in the sagmentation of the ovum. (From Gegenbaur's Comparative 



Anatomy.) 



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. 6). 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 

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

 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 mullic-n'ti lo</// or morn hi 

 stag*-. Tin 1 blastomeres next become arranged regularly in a 

 single layer the embryo assuming the form of a hollow sphere, 

 the blffxtiixpjHTc or llastula,with a wall composed of a single layer 

 of cells enclosing a cavity the scyniml.nlnm cavity or blastocaele. 



One side of the hollow blastula next becomes pushed inwards <> 

 invaginated, &s one might push in one side of a hollow indi 

 ball the result of this process of imagination, or fjastrul 



