572 SEGMENTATION OF OVUM CHAP. 



ovum. It follows from this that the essential nuclear matter 

 or chromatin of the oosperm often spoken of as the germ- 

 plasm and probably constituting the vehicle of heredity 

 is 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 ; 

 and as the reducing division and conjugation of the germ-cells 

 thus apparently render possible very varied permutations and 

 combinations of the hereditary qualities of both sexes, these 

 processes seem to have a great importance in connec- 

 tion with the phenomenon of individual variation (p. 

 216). 



Fertilisation being thus effected, the process of segmenta- 

 tion 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 the lancelet and 

 rabbit, for instance, which are each only yg- mm. (about 

 O^-Q- inch) in diameter, contain so comparatively small an 

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



