No. 593] THE MECHANISM OF CROSSING-OVER 



other; the other two threads, on the contrary, are sup- 

 posed not to have undergone recombination (' 'crossing- 

 over") and therefore would still lie across each other. 



It would seem equally possible, however, to interpret 

 these figures as meaning that (as shown in Fig. 9c and 

 9d) when the four threads began to separate into two 

 pairs, separation happened to start at some points (A and 

 C) between the identical halves and at other points (B) 

 between the homologous chromosomes, it being merely a 

 matter of chance in which way the separation started to 

 take place. It will be seen that this would result in the 

 formation of just such cross-figures, between two regions 

 where separation took place in opposite ways, as Jansseit> 

 finds. 



Another point in Janssens's evidence is that the pro- 

 phase chromosomes of maturation divisions not only show 

 the strands crossing, at points, but often bending in 

 towards each other near the middle, as though they had 

 formerly crossed there, and later undergone crossing-over. 

 It would seem possible, however, that this figure is merely 

 due to the chromosomes remaining in contact more closely 

 at the point where the spindle fiber is attached, and spread- 

 ing apart elsewhere,— a relation which figures of Bridges 

 and others show to exist between the two identical halves 

 of chromosomes in the prophases of oogonial mitoses. 



