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 always 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 split lengthwise, four 
strands are present (Fig. 47). If two of the strands 
fuse at the crossing place (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 Batracoseps, where he concludes from the 
