No. 593] THE MECHANISM OF CROSSING-OVER 



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one? Light might perhaps be thrown on the question by 

 a closer study of interference, and it was largely for this 

 reason that the experiment described in section V was 

 undertaken. If, for example, interference was a result 

 of length of loop (as would be true in schemes I and II), 

 and the length of the loop tended to vary more or less in 

 both directions, about a given mode, then coincidence 

 would be relatively higher between crossings-over which 

 were that distance apart, than between crossings-over 

 nearer together or still further apart. In other words, as 

 may be seen from Fig. 7, for small distances, the relative 



Y 



coincidence would be very small (interference high), for 

 longer distances much greater, and with still longer dis- 

 tances coincidence would fall again (interference would 

 rise). For distances double or triple the length of the 

 loop— if the chromosomes were as long as that— coinci- 

 dence would rise once more. Secondly, on the "loop ex- 

 planation" of interference just outlined, coincidence 

 should, at the modal distance, rise above the 100 per cent, 

 level, for crossing-over would occur at a given point (K) 

 more often in those cases when there is crossing-over at 

 another point (I) lying at the modal distance from K, than 

 in the average case. Of course it might be, however, that 

 there was no modal length of loop-that although short 

 loops were infrequent, all loops above a certain size were 

 equally frequent, or that the longer the loop, the more 



