258 RECENT CYTOLOGY 



method by which the reduction in the number of the 

 chromosomes is brought about. 



The simplest type of the process of reduction of the 

 chromosomes takes place at the formation of the male 

 germ-cells, or spermatozoa, of animals. For the sake of 

 clearness we shall consider the case of an animal in 

 which the somatic cells contain four chromosomes only, 

 and in which the reduced number characteristic of the 

 gametes is therefore two. 



The reduction in number of the chromosomes takes 

 place during two successive cell divisions which 

 immediately lead up to the formation of the germ- 

 cells. A particular mother-cell divides twice in rapid 

 succession, and the four cells thus arising develop into 

 spermatozoa without further subdivision. During 

 these two nuclear divisions the somatic number of 

 chromosomes becomes halved, giving rise to the 

 number characteristic of the gametes. 



Immediately before the first of these divisions the 

 chromosomes become closely associated together in 

 pairs, and in certain cases it has been shown that one 

 member of each pair is very probably the descendant 

 of a chromosome derived from the male parent, whilst 

 the other member of the pair is the descendant of the 

 corresponding maternal chromosome. 



This association of the chromosomes in pairs may 

 be so close, and may take place so early, that when 

 these bodies are first visibly differentiated only half 

 the usual number of them is to be seen. But in these 

 cases, too, it is reasonable to believe that each of 

 the chromosomes actually visible consists of a maternal 



