20 



ZOOLOGY 



cell, sjJO-m-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. mod.). The 



^projt 



seg.ruui 



Flo. 7. — Diagram illustrating the maturation and fertilisation of the ovum. A, formation of firpt 

 polar body ; B, beginning of fertilisation, sperms approaehing the mieropylc ; C, formation 

 of the male proimcleus ; D, api^roximation of the male and female pronuclei ; E, formation 

 of segmentation-nucleus; V cent, female centrosomc ; $ cent, male centrosome ; mem. egg- 

 membrane ; laicrop. micropyle ; poL polar bodies ; 9 p^oii. female pronucleus ; (5 pi-on. male 

 pronucleus ; se(r. iiucl. segmejitation 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 ^jrocess of fertilisation some attraction is 



