No. 593] THE MECHANISM OF CROSSING-OVER 289 



interference must have been very weak. Some illustra- 

 tions may make the meaning of this index clearer. If, 

 for example, coincidence is per cent, no double crossing- 

 over is occurring ; the interference between one crossing- 

 over and another is then complete. If coincidence is 45 

 per cent., this figure does not mean that 45 per cent, of 

 the individuals are double crossovers, but that 45 per 

 cent, of the number of double crossovers which would be 

 expected as a result of pure chance (whatever that num- 

 ber may have been) actually appeared, 55 per cent, hav- 

 ing been "interfered with," or somehow prevented from 

 occurring. If coincidence is 100 per cent., there has been 

 no interference, for the same number of double cross- 

 overs appeared as expected on the ground that the two 

 crossings-over did not interfere with each other's occur- 

 rence. 110 per cent, would mean that if one crossing- 

 over occurred, the other was 10 per cent, more likely to 

 occur than in cases of random distributions of crossings- 

 over. This would be " negative interference," for as 

 coincidence increases interference decreases. 



On Janssens's theory that crossing-over takes place in 

 the strepsinema stage, when the chromosomes are twisted 

 in loose loops, crossing-over would very seldom take 

 place at two points very near together, for this would re- 

 quire a tight twisting of the chromosomes. Accordingly, 

 on this theory interference was to be expected; further- 

 more it would be expected that interference was very great 

 between crossings-over that were in neighboring regions ; 

 but between crossings-over further apart there should be 

 little or no interference. The results were according to 

 this expectation; they indicated strongly that the inter- 

 ference was very great for crossings-over short distances 

 apart, but progressively diminished as the distances con- 

 sidered became greater. The conclusion drawn was that 

 crossing-over took place as postulated on Janssens's 

 theory, when the strands were loosely twisted in strep- 

 sinema, although the twisting and crossing-over did not 

 take place in the stereotyped manner suggested as a first 

 possibility, in the earlier part of this section. For there 



