554 F"e;rTI LIGATION citAP. 



fourth of the chromatin becomes enclosed as a nucleus, 

 which is distinguished as \}ne. female 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 some animals the first polar body has been found to divide after 

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

 ture ovum gives rise to a group of four cells — the mature ovum and 

 three polar-cells, just as the sperm-mothcr-cell gives rise to a group 

 of four cells, all of which, however, 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 actually conjugates : the others, like the drones in 

 a hive, perish without fulfilling the one function they are 

 fitted to perform. 



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

 angles to the surface of the egg and gradually passes 

 through the micropyle {microp) or works its way through 

 the vitelline membrane until its head lies within the egg- 

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

 panied by the intermediate piece or centrosome, penetrating 

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

 body, the 7nale pronucleus (c, $pron). 



The two pronuclei approach one another (d) and finally 

 unite to form what is called the segmentation-nucleus (e, seg. 

 nucl), the single nucleus of what is not now the ovum but 



