20 



ZOOLOGY 



cell, sperm-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 



microti 

 1*\ 



9 fir on 



segrtad 



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 ; B, formation 

 of segmentation-nucleus ; 9 cent, female centrosome ; 3 «»<• 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 



