CHAPTER VIII 



Fertilisation and Segmentation 



THE details of the process of fertilisation are hardly 

 less varied than those of the structure of the egg itself, 

 although its essential features are practically constant 

 throughout the animal kingdom. With rare exceptions the 

 egg is fertilised after it leaves the ovary, but the entrance 

 of the spermatozoon may take place in the oviduct or 

 uterus or after the egg is discharged, according to the 

 species. In some animals the spermatozoon enters before 

 the maturation (polar) divisions of the egg, in others after 

 the first division but before the second, and in others again 

 after both divisions are completed. Frequently only one 

 spermatozoon normally enters each egg, but in a number of 

 animals several constantly penetrate into the egg, although 

 only one conjugates with the egg-nucleus and the rest either 

 degenerate or in Elasmobranchs and Reptiles may form the 

 nuclei of the "vitelline syncytium" around the edge of the 

 germinal disc 1 . The normal entrance of more than one 

 spermatozoon occurs chiefly in eggs rich in yolk, for example 

 in Insects, Elasmobranchs, Reptiles, etc. In eggs which are 

 provided with an outer covering before they leave the ovary 

 the spermatozoon enters by the micropyle, in naked eggs 

 it can often penetrate the surface at any point, though there 

 may be a special region for its reception. It is uncertain 

 whether the spermatozoa are attracted to the egg by some 



1 In Bats, according to KOHLBRUGGE, several spermatozoa enter the egg, and 

 while one conjugates with the egg-nucleus others fuse with the nuclei of the 

 blastomeres during segmentation. This observation requires confirmation. 



