CHAP, xii FERTILISATION 571 



The first polar body has been observed to divide after 

 separating from the egg, so that the egg-mother-cell or 

 immature ovum gives rise to a group of four cells the 

 mature ovum, and three abortive ova or polar-cells, just as 

 the sperm-mother-cell gives rise to a group of four cells, all 

 of which, however, become sperms (Fig. 149). 



Fertilisation of the Ovum. Shortly after maturation, the 

 ovum is fertilised 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. 150, 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 centrosome (see p. 567), penetrating deeper 

 into the protoplasm, takes the form of a rounded body, 

 the male pronudeus (c, $pron). 



The two pronuclei approach one another (D) and finally 

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

 nuct)) the single nucleus of what is not now the ovum but 

 the oosperm\}\e impregnated egg or unicellular embryo 

 (compare pp. 197 and 198). 



The fertilising process is thus seen to consist essentially 

 in the union of two nuclear bodies, one contributed by the 

 male gamete or sperm, the other by the female gamete or 



