55 1 FKRTILIZATIOX citAi'. 



fourth of the chromatin becomes enclosed as a nucleus, 

 which is distinguished as \.\\ii fei)ialc pronucleus (b, ^ pron). 



The formation of both polar cells takes place by a 

 reducing division (p. 55 i) ; so that, while the immature ovum 

 contains double the number of chromosomes found in the 

 ordinary cells of the species, the mature ovum, like the 

 sperm, contains only one-half the normal number. 



In !Soinc animals the fir.sL ])i)lar body has been found Lu divide after 

 separating from the egg. In such cases the egg-mother-cell or iiniiia- 

 lure ovum gi\es rise to a group of four cells — the mature o\um and 

 three poUrr-cells, just as tlie sperni-mother-cell gives rise to a group 

 of f(.>ur cells, all of which, ln.iwever, become sperms (Fig. 139). 



Fertilization of the ovum. — Shortly after maturation, the 

 ovum is fertilized by the conjugation with it of a single 

 sperm. As we have found repeatedly, sperms are produced 

 in vastly greater numbers than ova, and it often happens 

 that a single egg is seen quite surrounded with sperms, all 

 apparently about to conjugate with it (Fig. 140, B). It has, 

 however, been found to be a general rule that only one of 

 these actuall)' conjugates : the others, like the drones in 

 a hive, perish without fullllling the one function they are 

 fitted to [lerform. 



The successful sperm (n) takes up a position at right 

 angles to the surface of the egg and gradual!)' passes 

 through the micropyle (niicrop) or works its way through 

 the vitelline membrane until its head lies within the egg- 

 protoplasm. The tail is then lost, and the liead, accom- 

 panied li\ the intermediate [jiece or centrosome, penetrating 

 deeper into the protoplasm, lakes on the form of a rounded 

 l)ody, the male proiiitcleiis (c, i /iroii). 



The lw(.i [pronuclei a|)proaih one antither (u) and fmally 

 unite to form what is called the seginenlatioii-iiiicleus (li, se-^. 

 unci), tile single nucletis of what is not now the ovidii but 



