1 70 CAUSES OF VARIATION 



At the proper stage each spermatocyte undergoes two divi- 

 sions (maturation divisions) into four cells, called spermatids, 

 each of which develops a tail and becomes functional, in which 

 finished condition it is known as a spermatozoon, when it is 

 ready to enter and fertilize the ripened ovum. 



The history and distribution of the chromatin matter in the 

 male is identical with that in the female, so that each sperma- 

 tozoon inherits one fourth the chromatin matter and one half tJie 

 chromosomes of the original cell. In plants the process differs 

 but the general results are the same. 



Significance of reduction. On the female side three fourths of 

 the chromatin matter has been extruded in the polar bodies, and 

 therefore lost to the line of descent. Whether reduction takes 

 place by extrusion or by rearrangement, one thing is certain : 

 when the second division is transverse, and possibly when it is 

 longitudinal, it results in an unequal division of physiological 

 units, if the identity of the chromosomes and the chromatin 

 granules has any meaning. If this division be anything else 

 than strictly qualitative, then the extrusion of the polar bodies 

 means a loss of something qualitative on the female side. 



On the male side the loss is not absolute, because all four 

 cells are functional, but if reduction has the -meaning we attach 

 to it, these four spermatozoa are not identical but different in 

 the hereditary substance with which they are provided. 



Significance of fertilization. Here, then, are two sexual cells 

 ready for union. Each has lost large portions of its chromatin 

 matter, the evident vehicle of transmission, and each brings to 

 the union but one half the number of chromosomes characteris- 

 tic of its species, strongly suggesting a loss of certain chromatin 

 granules and the hereditary qualities they represented. 



When fusion of the nuclei of these two germ cells takes place 

 at fertilization, however, the act of union again restores the full 

 and proper number of chromosomes, which will remain charac- 

 teristic of the new individual throughout its life, and which it 

 will hand down to posterity, always through the same compli- 

 cated method we have attempted to describe. The number of 

 chromosomes is evidently kept constant by the complicated 

 process of reduction during maturation and by fertilization 



