STRUCTURE AND PHYSIOLOGY OF ANIMALS 



23 



normal embryos. Such experiments on artificial 'parthenogenesis, 

 as it is termed, show that the entry of a male cell into the ovum 

 is not necessary for the development of the embryo even in cases 

 in which gamogenesis is normal ; but that other exciting influences 

 may bring about the same result. 



Though, as stated above, the female pronucleus, under normal 

 circumstances, plays so important a role in the development, it 

 has been shown that it can be dispensed with. When unfertilised 

 ova of a sea-urchin are broken up, and fragments devoid of 

 nuclei are placed in water along with sperms, the fragments may 

 be fertilised ; and, the nucleus of the sperm taking the place 

 of the segmentation-nucleus, normal young, differing from those 

 produced in the usual manner only in their smaller size, may 

 be developed. This phenomenon is known as merogony. 



The result of fertilisation is the formation of the impregnated 

 ovum, or oosperm as it is called. The oosperm, it is to be noted, 

 before development begins, consists in general of the primary 

 ovum minus the portions of the substance of its nucleus removed 



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



Comparative Anatomy.) 



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 

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

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

 contains comparatively little or no food-yolk, consisting mainly 

 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 w r hich are termed the 

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

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



