1352 



PHYSIOLOGY 



pairs and then divide as in the spermatocytes, so that each of the 

 daughter cells contains one half the somatic number of chromosomes. 

 The larger of the two resulting cells, which retains most of the cyto- 

 plasm, 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 homo- 

 typical 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. 556). The mature ovum has, however, only half the normal 

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



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). 



. Growth-period. 



M aturation-period. 



Fie. r 556. 



forming the head of the spermatozoon. The only difference therefore 

 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 

 abortive, whereas in the spermatozoon all four daughter cells pro- 

 duced from the spermatocyte remain functional. The production of 

 these two kinds of sexual cell is represented in Figs. 555 and 556. 



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 echinoderms. 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 



