GENERAL OUTLIAE 



175 



in the most general terms, this parallel is as follows : ^ In both sexes 

 the final reduction in the number of chromosomes is effected in the 

 course of the last two cell-divisions by which the definitive germ-cells 

 arise, each of the four cells thus formed having but half the usual 

 number of chromosomes. In the female but one of the four cells 

 forms the "ovum" proper, while the other three, known as t\\Q. polar 

 bodies, are minute, rudimentary, and incapable of development (Figs. 

 64, 71, 86). In the male, on the other hand, all four of the cells become 

 functional spermatozoa. This difference between the two sexes is 

 probably due to the physiological division of labour between the germ- 

 cells, the spermatozoa being motile and very small, while the q^^ 

 contains a large amount of protoplasm and yolk, out of which the 



Primordial germ-cell. 



Oogonia. 



Primary oocyte 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. 



" Maturation-period. 



Fig. 87. — Diagram showing the genesis of the egg. [After Boaeri.] 



main mass of the embryonic body is formed. In the male, therefore, 

 all of the four cells may become functional ; in the female the func- 

 tions of development have become restricted to but one of the four, 

 while the others have become rudimentary (cf. p. 182). The polar 

 bodies are therefore to be regarded as abortive eggs — a view first put 

 forward by Mark in 1881, and ultimately adopted by nearly all inves- 

 tigators. 



I. Reduction in tJie Female. Formation of the Polar Bodies 



As described in Chapter III., the Qgg arises by the division of cells 

 descended from the primordial egg-cells of the maternal organism, and 

 these may be differentiated from the somatic cells at a very early 



^ The parallel was first clearly pointed out by Plainer in 1889, ^f^' was brilliantly demon- 

 strated by Oscar Hcrtwig in the following year. 



