INTRODUCTORY 



takes place (or at any rate begins) in the reproductive organ or 

 gonad (ovary or testis according to the sex of the parent) and 

 consists essentially in the reduction of the nuclear material to 

 one-half of what it was in the unripe ovum or spermatozoon. 



1. Polar bodies 

 Fen}nle pronucleus 



Male 'pronucleus 



Female ^^ronucleus 



Male pronucleus ^ ^^' 



Scffmentation 

 nucleus 



2. 



Female pronucleus 

 Male pronucleus 



Fused pronuclei 



Segmentation 



jiHcleus 

 {commencing 



division) 



Fig, 5. — FertiHsation of the ovum of the mouse. The male pronucleus 

 represents the head of the spermatozoon, and the female pronucleus 

 is the nucleus of the ovum after the polar bodies (see p. 37) have 

 been given off. (After Sobotta.) (From Grmfs Ana torn//.) 



The process, which is described in some detail in a later chapter, 

 may be regarded as an act of preparation for fertilisation Avhen 

 the two gametes come together and their respective nuclei also 

 fuse, the nuclear material being thereby restored to wdiat it was 

 originally in the unripe gamete. 



There is evidence that in some animals at least the ova and 

 spermatozoa have a definite attraction for one another (Dakin 



