LINKAGE 59 
Non-crossovers Crossovers ; 
White round Red bar White bar Red round 
28 per cent. 28 per cent. 22 per cent. 22 per cent. 
56 per cent. 44 per cent. 
Here a large amount of crossing over appears, about 
44 per cent. In fact, so freely do the factors inter- 
change that without sufficiently large and accurate 
numbers the linkage might entirely escape detection. 
THE MECHANISM OF CROSSING OVER 
If it be admitted that the Mendelian factors are 
carried by chromosomes it can not be denied that 
interchange between homologous chromosomes must 
occur, for sex linked factors cross over from each 
other, and yet are known to be in the same pair of 
chromosomes, since they all follow the X chromo- 
some in its distribution. The evidence allows for no 
other interpretation. But why should crossing over 
take place so rarely between certain factors and so 
often between others? We can make use here of 
certain information in regard to the chromosomes 
that gives a very simple answer to the question. In 
the early germ cells, before the maturation period 
begins, the chromosomes appear to be scattered in 
the nuclei, and the homologous chromosomes in 
many cases show no tendency to lie together, although 
in some animals, e.g. in many flies, the members of a 
pair are often found side by side. In thisearly period 
the germ cells divide as do other cells and thereby 
increase in numbers. But at the termination of this 
