THE CHROMOSOMES 



133 



method by which the strands are found joined at the 

 time when they draw apart, that cross union of the 

 threads must have previously taken place. 



If crossing over be supposed to take place between 

 two single threads (Fig. 24) all four gametes that 

 ultimately result from such a cell will be crossover 

 gametes. On the other hand, if crossing over takes 



^ B c D 



Fig. 47. — Four stages in crossing over, according to the "typical" 

 chiasma type of Janssens. The white rod and the black rod are each 

 split lengthwise; crossing over takes place only between two of the four 

 strands. 



place by means of the chiasmatype (Fig. 47) only 

 two of the resulting four cells will be crossover 

 gametes, the other two being non-crossover gametes.^ 

 Looked at from the point of view of the total 

 output, there would be no way in which to tell 

 whether one or the other of the above processes has 

 taken place; although the formation of a given 

 number of crossover gametes involves only half as 

 many participating cells in the case of the single 

 thread type as in the case of the double thread type. 



^ If, after the thick threads have split, crossing over involving hoih 

 strands of each chromosome should take place, instead of only one 

 strand as in the chiasmatype, sensu strictu, the four gametes that result 

 would be crossover gametes. 



