110 METAZOA IN GENERAL 



135. Oogenesis. — In oogenesis, also, after the periods of multiplica- 

 tion and growth have been completed, the primary oocyte undergoes a 

 first maturation division in which, in contrast to what occurs in sperma- 

 togenesis, it is very unequally divided (Fig. 42). One of the two daughter 

 cells is small and, while it contains half the nuclear material, has prac- 

 tically none of the cytoplasm. The other is much larger, receiving in 

 addition to half the nuclear material practically all of the cytoplasm. 

 The smaller daughter cell is termed the first polar body; and the larger 

 one, the secondary oocyte. Following this division a second division of 

 the first polar body may occur, giving rise to two smaller polar bodies 

 each equal in size to half of the first polar body. The secondary oocyte 

 undergoes another unequal division, a second polar body being formed as 

 before with very little cytoplasm, while the larger cell is known as the 

 egg cell. As in spermatogenesis one of these two divisions is a reduction 

 division in which the number of the chromosomes is reduced to the hap- 

 loid number. The result of oogenesis, therefore, is to produce one large, 

 functional egg cell and either two or three polar bodies depending upon 

 whether or not the first polar body undergoes division. These polar 

 bodies die, disintegrate, and disappear. In effect, all of the cytoplasm 

 which would have gone to four cells if the divisions of the cells had been 

 equal has been accumulated in the one. This egg cell becomes still 

 larger by the accumulation within it of yolk and thus becomes fully 

 mature. This accumulation of yolk in the mature egg cell is to provide 

 the necessary food supply for the embryo which will develop from it until 

 the developing individual can secure food for itself. 



It should be observed that in both oogenesis and spermatogenesis all 

 of the cell divisions except the reduction division are mitotic. 



136. Comparison and Contrast between Spermatogenesis and 

 Oogenesis. — It is clear from the description of the two processes that 

 there are many ways in which they are ahke; the more important simi- 

 larities may be enumerated as follows: 



1. Both start with a primordial germ cell. 



2. Both pass through three periods, namely, multiplication, growth, 

 and maturation. 



3. Both undergo a process known as synapsis in the growth period. 



4. Both possess two maturation divisions. 



5. Both exhibit a reduction in the number of chromosomes. 



On the other hand, the two processes are sharply contrasted in several 

 ways: 



1. Spermatogenesis results in the production from each spermato- 

 gonium of four similar sperm cells, all of which are functional, while 

 oogenesis results in the formation from each oogonium of only one large 

 cell, the egg cell, and of three small nonfunctional cells or polar 

 bodies. 



