572 FERTILISATION CHAP. 



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 which develop no further, 1 

 just as the sperm-mother-cell gives rise to a group 

 of four cells, all of which, however, become sperms 

 (Fig. 150). 



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. 151, 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 works 

 its way into the ovum, passing through the micropyle 

 (microp) when present, until its head lies within the 

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

 accompanied by the centrosome (see p. 568 and Fig. 148), 

 penetrating deeper into the protoplasm, takes the form 

 of a rounded body, the male pronucleus (C, 3 pron) . The 

 centrosome of the ovum disappears, that of the sperm 

 dividing subsequently. 



The two pronuclei approach one another (D) and 

 finally unite to form what is called the conjn^i Hon- 

 or segmentation-nucleus (E, seg. nucl), the single nucleus 

 of what is not now the ovum but the oosperm the 



1 It will be remembered that there is an analogous casting off 

 of nuclear material in the process of conjugation in Paramcecium 

 (p. 268). 



