MATURATION 147 



of the deutoplasmic substance. The equal division of the 

 nucleus is thus accomplished without appreciable loss from the 

 oocyte of any of the cytoplasm and food reserve. 



The second maturation or homotype division follows, either 

 at once, or after a long pause in cases where the sperm normally 

 enters at this time. The figure for the second maturation 

 division is either a new figure or the reorganized remains of the 

 preceding. In either case it appears in the region occupied by 

 its predecessor, often in a radial position from its first appear- 

 ance (Figs. 76, 77). Again the division is very unequal and the 

 secondary oocyte gives rise to the large mature ovum and a 

 small second polar body, again alike as regards nuclear com- 

 position. The first polar body may or may not divide into 

 two at the same time; we may assume that such a division is 

 normal, but on account of the degeneration of the polar bodies 

 such a division tends to disappear; in different forms various 

 stages of this division are reached. In a few rare instances, 

 as in some Rotifers and Insects, the second polar body may also 

 divide. 



The chromosomes remaining in the ovum then re-form a 

 reticular nucleus, smaller than the original oocyte nucleus, and 

 with the haploid number of chromosomes; after forming a thin 

 membrane the nucleus moves toward the cytoplasmic center 

 of the ovum and there awaits union with the nucleus of the 

 fertilizing spermatozoon (Figs. 76, 77). With few if any ex- 

 ceptions, the centrosome of the ovum is entirely lost as the 

 nucleus is reconstructed; the absence of the centrosome is one 

 of the peculiarities of the egg cell. It should be added that 

 when the maturation is not completed until after the entrance 

 of the sperm, the egg does not ordinarily re-form a typical 

 nucleus but proceeds at once to unite with the sperm nucleus. 



The final result of the two maturation divisions of the 

 primary oocyte is the formation of four cells whose nuclei are 

 similar, and which are morphologically exactly equivalent to 

 one another (Figs. 74, 78). Physiologically, however, there is 

 the greatest difference among them, since of the four only one, 

 the ovum, is able to function, or indeed even to remain alive, 



