132 THEORY OF EVOLUTION 



chromosome comes to be now on one side and 

 now on the other side of its mate. 



When the chromosomes separate at the next 

 division of the germ cell the part on one side 

 passes to one pole, the part on the other to the 



FIG. 64. Scheme to illustrate a method of crossing over of 

 the chromosomes. 



opposite pole, (figs. 64 and 65). Whenever 

 the chromosomes do not untwist at this time 

 there must result an interchange of pieces 

 where they were crossed over each other. 



Janssens has found at the time of separation 



