CROSSING-OVER 117 



same kind. It has been observed that as the members 

 of a pair come together they occasionally twist 

 aromid each other (fig. 51). If where they overlap 

 thev should break and the ends uHite with the cor- 

 responding ends of the opposite chromosomes (fig. 

 52), the conditions of crossing-over would be ful- 

 filled. 



Fig. 52. — Diagram to illustrate crossing-over of two con- 

 jugating threads. 



Unfortunately the evidence that crossing-over 

 takes place at the time of maturation, as the result of 

 overlapping of the chromosomes, is very meagre and 

 by no means conclusive, nevertheless, as far as it 

 goes, this evidence is favorable for such an interpre- 

 tation of genetic crossing-over as that given above. 



From the genetic evidence for crossing-over it is 

 possible to determine the relative location of the 

 genes in the chromosomes. The method can not be 

 given here in detail but the general point of view 



