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 
