THE MECHANISM OF HEREDITY 



41 



per cent. In the Drosophila male, no interchange at all 

 takes place. 



In 1909 Janssens published a detailed account of a 

 process that he called Chiasmatypie. Without entering 

 here into the details of Janssens' work it may be stated 

 that he brought forward evidence which he believed to 

 show that there is an interchange of blocks or segments 

 between the members of the conjugating pairs of chromo- 

 somes which is traceable to an earlier twisting of the 

 two conjugating chromosomes around each other (Fig. 

 26). 



Fig. 26. 

 The conjugation of the chromosomes in Batrachoseps. The twisting 

 of the two thin threads around each other is suggested in one of 

 the two chromosomes in the middle figure. (After Janssens.) 



Unfortunately there is scarcely any stage in the matu- 

 ration divisions that is as much in dispute as this one 

 involving the twisting of the chromosomes. From the 

 nature of the case it is practically impossible to demon- 

 strate, even when twisting of the chromosomes is ad- 

 mitted, that it actually leads to an interchange of the 

 kind demanded by the genetic evidence. 



