THE CHROMOSOMES 165 



series of beads (chromomeres) , then, when the con- 

 jugating chromosomes twist around each other, 

 whole sections of one chain will come to lie, now on 

 one side, now on the other side, in the double chro- 

 mosome. If, when the two series of beads come to 

 separate from each other, all of the segments that lie 

 on the same side tend to go to one pole, and all of 

 those on the opposite side to the other pole, each 

 series must, in order to separate, break apart between 

 the beads at the crossing point. Moreover, since 

 the essential part of the process is that homologous 

 beads go to opposite poles it follows that the break 

 between the beads of two chains must ahvays be at 

 identical levels. It is not necessary to assume that 

 crossing over takes place at every node, but only that 

 it may sometimes take place. In fact, our work on 

 Drosophila shows for the sex chromosome in the 

 female that crossing over takes place in only about 

 half of the cells, and double crossing over is a rather 

 rare event. 



There is a later stage also at which crossing over 

 might be supposed to take place. After the thin 

 threads have conjugated to form the thick threads, 

 and these have shortened and sj^lit lengthwise, four 

 strands are present (Fig. 47). If two of the strands 

 fuse at the crossing jilace (the pieces of one strand 

 uniting endwise with the pieces of the other) crossing 

 over is brought about. It is this type in particular 

 that Janssens named chiasmatype. In support of 

 this method of crossing over are Janssens' observa- 

 tions on Batracoscps, where he concludes from the 



