X( >OLOGY 



SECT. 



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, OT 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, 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. 6). 

 This either affects the entire substance (holobkistic or complete 

 segmentation) or only a part (meroblostic 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 



':. 6. Various stages in the segmentation of the ovum. (From Gegenbaur's Compant,,, 



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, b}' the proc. sss 

 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 

 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 mnUirrry lm/// or morn la 

 stage. The blastomeres next become arranged regularly in a 

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

 the blastosphere or blastula,with a wall composed of a single layer 

 of cells enclosing a cavity the segmentation <"///// or lil<i*toc,,'lc. 



One side of the hollow blastula next becomes pushed inwards or 

 invaginated, as one might push in one side of a hollow india-rubber 

 ball the result of this process of invagination, or <!<!*i rulitn>n as 



