No. 592] THE MECHANISM OF CBOSSING-OVER 221 



less and bent, no recombinations between them have so 

 far been obtainable. Group IV, therefore, forms a marked 

 contrast to all the other groups as regards the frequency 

 of separation within it, and this result is the more strik- 

 ing, not only because it shows that there is a group of 

 factors corresponding in separation frequency to the pair 

 of short chromosomes, but also because it happens that 

 this group is the same one as that which had previously 

 been identified with the pair of short chromosomes by 

 reason of the fewness of the mutant factors discovered 

 in it. 



It is therefore evident, not only that the relative sizes 

 of the chromosomes are in a general way like the separa- 

 tion frequencies of the groups, but also that where there 

 is evidence from another source indicating in which 

 chromosome a certain group lies, this is the very one to 

 which the group corresponds by its total frequency of 

 separation. It has been shown that this is true in the 

 case of the fourth group. In the case of the first group, 

 the sex-linked inheritance of the latter connects it with 

 the X-chromosome, and since this is the moderately long 

 chromosome, it is just this one with which group I would 

 be identified by its frequency of separation. The other 

 two groups, both of which are long— one certainly very 

 long, and the other probably so -are thus left to corre- 

 spond with the remaining chromosomes, both of which 

 are very long and indistinguishable in appearance. 



In the remainder of this article, therefore, the word 

 ' ' chromosome" will be used instead of "group" and 

 "crossing-over" instead of "separation of linked 

 factors." 



(To be continued) 



