i STRUCTURE AND PHYSIOLOGY OF ANIMALS lf 



nucleus (g pron.} coalesces with the female pro-nucleus to form a 

 single nucleus called the segmentation nucleus (scg. nucl.}. The 

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



B 



mem, 



icenl 



D 



mem 



FIG. 5. Diagnim illustrating the maturation and fertilization of the ovum. A, formation of fii>t 

 polar globule ; B, beginning of fertilization, spermatozoa approaching the micropyle ; C. for- 

 mation of the male prouueleus ; I), approximation 'of the male and female proimclei ; E. for- 

 mation of segmentation-nucleus ; 9 cro<. female centrosome ; $ ee.i. male centrosome ; mem, 

 egg-membrane; nurroji. micropyle ; pol. polar bodies ; $ fit-on, female pronucleus ; 3 pron. 

 male pronucleus ; .<>;/. ,ti>ct. segmentation nucleus. 



two nuclei; but the centrosomes, one (^ cent.} derived from the 

 sperm, and the other ( o cent.} from the ovum, also seem to take a 

 share. Sometimes each of these divides into t\v<>, and the two 

 centrosomes resulting from the division of that derived from the 



C 2 



