xii SEGMENTATION OF OVUM 573 



impregnated egg or unicellular embryo (compare pp. 197 

 and 198). 



The fertilising process is thus seen to consist essentially 

 in the intermingling in equal proportions of chromatin- 

 material derived from two conj ugating cells or gametes, 

 and the term is often restricted to those cases of conjuga- 

 tion in which the gametes differ from one another, so 

 that one can distinguish between a male gamete and a 

 female gamete, known in the Metazoa as a sperm and 

 an ovum respectively. It follows from what has been 

 said above that the essential nuclear matter or chromatin 

 of the oosperm often spoken of as the germ-plasm and 

 probably constituting the material basis of heredity is 

 derived in equal proportions from each of the two 

 parents. Moreover, as both male and female pronuclei 

 contain only half the number of chromosomes found in 

 the ordinary cells of the species, the union of the pro- 

 nuclei results in the restoration of the normal number 

 to the oosperm ; and as the reducing divisions 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 connection with the 

 phenomenon of individual variation (p. 216). 



Fertilisation being thus effected, the process of seg- 

 mentation or division of the oosperm takes place as 

 described in previous chapters. 



Different Types of Ova and of 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 



