THE CELLULAR BASIS 137 



chromosomes form a closely wound coil of 

 long chromatin threads (Fig. 31), and when 

 these threads uncoil later it is seen that the 

 chromosomes have united in pairs (Figs. 31 D 

 and E, 32 B } 33 B) ; this process is known as 

 synapsis, or the conjugation of the chromo- 

 somes, and there is evidence that one member 

 of each synaptic pair is derived from the 

 father, and the other from the mother. The 

 union of these chromosomes is probably not 

 so close that they lose their identity, though 

 there may possibly be some interchange of 

 substance between them. By this union of the 

 chromosomes into pairs the number of sepa- 

 rate chromosomes is reduced to half the 

 normal number ; if there are usually 4 chromo- 

 somes, as in Ascaris, they are reduced to 2 

 pairs ; if 48 chromosomes, as in man, there are 

 24 of these pairs.* 



In the conjugation of the chromosomes it 

 is plain that, generally speaking, those 

 chromosomes which are similar in shape and 

 size unite; big chromosomes unite with big 

 ones, little ones with little ones, and those of 

 peculiar shape with others of similar shape 



* See footnote p. 153. 



