20 



ZOOLOGY 



SECT. 



cell, spcmn-cell, or sperm, penetrates into the interior of the female 

 cell or ovum, and the nucleus which it contains the male pro- 

 nucleus (C, $ pron.) coalesces with the female pronucleus to form a 

 single nucleus called the segmentation nucleus (E, seg. nucL). The 



pol 



ment 



seg ruuL 



FIG. 7. Diagram illustrating the maturation and fertilisation of the ovum. A, formation of first 

 polar body ; B, beginning of fertilisation, sperms approaching the micropyle ; C, formation 

 of the male pronucleus ; D, approximation of the male and female pronuclei ; E, formation 

 of segmentation-nucleus ; 9 cent, female centrosome ; <$ cent, male centrosome ; mem. egg- 

 membrane ; microp. micropyle ; pol. polar bodies ; ? pron. female pronucleus ; pron. male 

 pronucleus ; seg. nucl. segmentation nucleus. 



principal part in the process of fertilisation is thus played by the 

 two nuclei. The female centrosome disappears: a male centrosome 

 enters with the sperm. 



Apparently in this process of fertilisation some attraction is 



