STRUCTURE AND PHYSIOLOGY OF ANIMALS 



19 



nucleus (3 pron.) coalesces with the female pro-nucleus to forma 

 single nucleus called the segmentation nucleus (seg. nucl.). The 

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



FIG. 5. Diagram illustrating the maturation and 'fertilization of the ovum. A, formationTof first 

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

 mation of the male proimcleus ; D, approximation of the male and female pronuclei ; E, for- 



mation of segmentation-nucleus ; $ ceii-t. female ceiitrosome ; 3 cent, male centrosome ; mem. 

 egg-membrane; microp. micropyle ; pol. polar bodies ; $ pron. female pronucleus; cJ pron. 

 male pronucleus ; seg. nucl. segmentation nucleus. 



two nuclei; but the centrosbmes, one (<j 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 two, and the two 

 centrosomes resulting from the division of that derived from the 



c 2 



