ESSENTIAL FEATURES OF SEXUAL PROCESS 



1209 



is still called the ovum, while the smaller one is spoken of as the ' first polar 

 body.' The ovum now divides again and throws off a second polar body, 

 the division being of the homotypical variety. The first polar body may 

 also divide, so that from the original ovum three cells are produced, one of 

 which retains the greater part of the cytoplasm, while the others are extruded, 

 and degenerate (Fig. 558). The mature ovum has, however, only half the 

 normal number of chromosomes, so that its nucleus is equivalent to the 

 nucleus forming the head of the spermatozoon. The only difference there- 

 fore between the formation of ovum and spermatozoon is that in the former 

 case three of the cells formed by the division of the primitive ovum are 



Primordial germ-cell. 



Oogonia. 



Primary obcyte or ovarian egg. 



Secondary oocytes (egg and 



first polar body ) 



Mature egg and three polar bodies 



Division-period (the number of divi- 

 sions is much greater). 



G rowtn-perioa. 



Maturation-period. 



FIG. 558. 



abortive, whereas in the spermatozoon all four daughter cells produced 

 from the spermatocyte remain functional. The production of these two 

 kinds of sexual cell is represented in Figs. 557 and 558. 



Since the nuclei of the mature ovum and spermatozoon only contain half 

 the normal number of chromosomes, they are generally spoken of as pro- 

 nuclei. 



FERTILISATION 



The essential features of fertilisation, i.e. the union of the sexual cells, 

 are best studied in some of the lower invertebrates, such as ascaris or echino- 

 derms. In the latter fertilisation takes place in the sea- water, into which 

 both ova and spermatozoa are extruded. The ovum of the echinoderm 

 consists of a naked mass of protoplasm. Of the countless hordes of sperma- 

 tozoa which may be in the neighbourhood of a given ovum only one as a 

 rule enters. As soon as the spermatozoon has entered the ovum a tough 

 membrane is rapidly formed round the latter, so preventing the entrance of 

 any further spermatozoa. The head of the spermatozoon enters the egg, 

 while the tail atrophies and disappears. The head of the spermatozoon 

 enlarges and assumes the character of a nucleus, the dense mass of chro- 

 matin breaking up first into a thread and then into the characteristic number 



