1260 



PHYSIOLOGY 



the heterotypical division, the chromosomes fuse in pairs and then divide as in the sper- 

 matocytes, 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 



Primordial germ, cell 



Spermatogonia. 



Primary spermatocyte. 

 Secondary sperrnatocytes. 



Spermatids. 

 Spermatozoa. 



Division .period (the number of divi 

 sions is much greater) . 



Growth -period. 



Maturation period. 



FlQ. 570. 



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. 571). 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 



Primordial germ cell. 



OogonJa. 



Primary oBcyte or ovarian egg. 



Secondary oocytes (egg and 



first polai body) 



Mature egg and three polar bodies 



Division period (the number of divi 

 sions is much greater). 



Growth period. 



Maturation.period. 



FlO. 571. 



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 produced from the spermatocyte remain 

 functional. The production of these two kinds of sexual cell is represented in Figs. 570 

 and 571. 



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

 number of chromosomes, they are generally spoken of as pro-wide i. 



