Fertilisation of Egg. 125 



As to sperm formation, the details vary in different 

 animals, but an instance selected will serve to illustrate the 

 facts upon which it is necessary to lay stress. The nucleus of 

 the sperm mother cell undergoes what is called a " reducing 

 division ; " that is to say, there are at first twenty-four chromo- 

 somes, which are divided into two lots of twelve each when the 

 original cell has divided into two ; in the product of the next 

 division each of the four cells formed has a nucleus containing 

 only six chromosomes. Now, when this state of affairs is com- 

 pared with what occurs in the ovum, it is plain that it is not 

 strictly comparable with the division of the ovum after fertilisa- 

 tion, for when that occurs each chromosome in the nucleus 

 divides by splitting, so that each daughter nucleus has the same 

 number of chromosomes as the parent nucleus, though each 

 separate chromosome is but half that of the parent nucleus. 

 But during the formation of the polar bodies events occur 

 which are exactly comparable to that of the sperm cells. Each 

 of the two polar bodies is formed by a reducing division. 

 What has happened in the ovum, therefore, is a division of the 

 egg mother cell into three or four daughter cells, of which, 

 however, only one becomes an ovum. In the case of the 

 sperm all the daughter cells become spermatozoa. 



Fertilisation of the Ovum. 



The mature spermatozoon is a tadpole-like body with a 

 head " resembling a conical bullet,'' a neck composed of a 

 small sperule, and a long vibratile tail. The head consists of 

 nuclein. By the active lashing of the tail the spermatozoon 

 approaches the ovum — which in the starfish (see Fig. 61), a 

 classical object for the study of the processes of fertilisation, 

 protrudes a little hillock of protoplasm to meet it — and bores 

 its way into the interior, the tail in some cases remaining behind. 

 Occasionally (Fig. 63) more than one spermatozoon enters the 

 ovum. Directly it enters, the ovum secretes a delicate membrane, 

 which prevents the entrance of another spermatozoon. The 

 head of the spermatozoon (composed, it will be remembered. 



