xxiv FUSION OF PRONUCLEI 263 



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. 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 (A, sp) takes up a position at right 

 angles to the surface of the egg and gradually works its way 

 through the vitelline membrane until its head lies within the 

 egg protoplasm (D, sp). The tail is then cast off, and the 

 head, penetrating deeper into the protoplasm, takes on the 

 form of a rounded nucleus-like body, the male pronucleus 

 (E 1 , pr. nit. $ ). 



The two pronuclei, each accompanied by its directive 

 sphere and centrosome, approach one another (E 1 , E 2 ) and 

 finally unite to form the single nucleus (r 1 F R ) of what is 

 now not the ovum but the oosperm the impregnated egg or 

 unicellular embryo. The fertilizing process is thus seen to 

 consist of the union of two nuclear bodies, one contributed 

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

 gamete or ovum. It follows from this that the essential 

 nuclear matter or chromatin of the oosperm is derived in 

 equal proportions from each of the two parents. 



Moreover, as both male and female pronuclei contain only 

 half the number of chromosomes found in the ordinary cells 

 of the species, the union of the pronuclei results in the 

 restoration of the normal number to the oosperm. 



There is reason for thinking that the directive spheres of 

 the sperm and ovum as well as their nuclei unite with one 

 another : in this way the directive sphere of the oosperm 



