30 HEREDITY 



forms — may actually give rise, unaided, to a new 

 individual : and there is some evidence to show 

 that the spermatozoon has a similar potentiality. 



Now let us recall what was said as to the typical 

 structure of a cell. We saw that the cell nucleus 

 contains a certain stainable structure called 

 chromatin. Now, when any cell divides, it is 

 found that the chromatin breaks up into a number 

 of separate portions called chromosomes.^ The 

 number of these chromosomes is invariable for every 

 cell of any species of animal or plant. If eight 

 or sixteen be the number, eight or sixteen re- 

 spectively are always observed in the division of 

 any cell of any part of the body. 



Let us suppose, then, that sixteen is the charac- 

 teristic number of the species we are considering. 

 The act of bi-parental reproduction consists in 

 the intimate blending of the nucleus of the ovum 

 with the nucleus of the spermatozoon. The result 

 would be the formation of a new cell — the cell from 

 which the new individual is to arise — which would 

 contain thirty-two chromosomes in its nucleus ; 

 and at every generation the number of chromosomes 

 proper to the species would be doubled. But a re- 

 markable fact was discovered by Van Beneden. He 

 found that every ovum and spermatozoon is formed 

 by the division of what is called a mother-germ-cell 

 (male or female). This division is of a unique 

 character ; for it results in the formation of cells — 

 the actual ova or spermatozoon — which contain 

 only one-half the number of chromosomes charac- 



1 Not to be confused with the centrosome, which usually lies 

 near the nucleus. 



