THE MECHANISM OF HEREDITY 



39 



not demonstrate completely that the chromosomes remain 

 intact throughout their history, the evidence, as far as it 

 goes, is favorable to this view. 



There is, however, a very important limitation that 

 must be placed on this statement. The genetic evidence 

 clearly proves that between the members of the same pair 

 of chromosomes there is at times an orderly interchange 

 of parts. Does the cytological evidence show any indica- 

 tion of sucli an interchange 1 Here we enter on more ques- 

 tionable ground. 



a b c d 



Fro. 25. 

 The nuclei of four pairs of sister cells (above and below) show- 

 ing the position of the daughter chromosomes as they come out of 

 the resting nuclei. (After Boveri.) 



Mechanism of Crossing-Over. 



If, as other evidence clearly shows, the chromosomes 

 are the bearers of genes, and if the genes may inter- 

 change between members of the same pair, it follows that 

 sooner or later we may expect to find some kind of mecha- 

 nism by which such interchange takes place. 



Several years before the genetic discovery of crossing- 

 over, the process of conjugation of the chromosomes, and 

 their reduction in number in the mature germ cells had 

 been fully established. It was demonstrated that at the 



