XI SEGMENTATION OF OVUM 555 



the oosperm — the impregnated egg or unicellular embryo 

 (compare pp. 197 and 198). 



The fertilizing process is thus seen to consist essentially 

 in the union of two nuclear bodies, one contributed by the 

 male gamete or sperm, the other by the female gamete or 

 ovum. It follows from this that the essential nuclear matter 

 or chromatin of the oosperm — often spoken of as the gerin- 

 plasm—As, derived in equal proportions from each of the two 

 parents. Moreover, as both male and female pronuclei con- 

 tain only half the number of chromosomes found in the ordi- 

 nary cells of the species, the union of the pronuclei results 

 in the restoration of the normal number to the oosperm. 



Fertilization being thus effected, the process of segmenla- 

 tioit or division of the oosperm takes place as described in 

 previous chapters. 



Different types of ova and their segmentation, — Before 

 passing on to consider further details in the process of 

 development of the oosperm, we must briefly refer to some 

 differences already noted in the ova of different animals. 



We have seen that in all cases the immature egg is a 

 simple, minute cell, but that owing to the deposition of 

 yolk-granules in its protoplasm, it may reach a compara- 

 tively large size {e.g. Crayfish, Dogfish, Bird). The pres- 

 ence of a greater .or less amount of yolk in the ovum 

 results, as we know, in very considerable differences as 

 regards its mode of segmentation, as well as in its subsequent 

 development. The minute eggs of Amphioxus and the 

 Rabbit, for instance, which are each only -^'^ mm. (about 

 ji^ inch) in diameter, contain so comparatively small an 

 amount of food-yolk as not to interfere materially with the 

 process of segmentation : such ova are called akcithal. 

 AVhen the quantity of food yolk is relatively greater, it may 



