126 



Elementary Zoology. 



of nuclein) is termed the male pronucleus. In front of it, as it 

 pushes its way towards the female pronucleus (original nucleus 

 of egg-cell minus the polar bodies), is a small clear body 

 round which the granules of the egg protoplasm are beginning 

 to arrange themselves in a radiating fashion, which is the 

 product of the neck of the spermatozoon, and has been called 

 the male centrosome. The female pronucleus has a correspond- 

 ing centrosome. The two pronuclei now approach more 



^/■■■l"-- 



Fig. 62. — Fertilization of the ovum of an Echinoderm. From Quain's " Anatomy, "' 

 after Selenka.) 



•S", spermatozoon ; m.^r., male pronucleus : /.pr., female pronucleus- 



rapidly, and fuse together. The centrosomes each divide into 

 two ; each half unites with the half of the other. Their move- 

 ments have been fancifully termed " the quadrille of the 

 centrosomes." The definitive nucleus is now formed, and it 

 has, it will be observed, a centrosome at each end. 



Thus the process of fertilisation is essentially the union of 

 the nuclei of two dissimilar but homologous cells, of which one, 

 the male, is small and active, the other, the female, is large 

 and passive. 



Division of the Ovum. 



The fertilised ovum now proceeds to divide into two. 

 This process is initiated by the nucleus ; but the modus 



