29"2 



THE AMERICAN NATURALIST 



[Vol. L 



interpretation. If crossing-over occurred during the 

 "amphitene" stage, or not long after, would there be any 

 possible explanation of the fact that one point of crossing- 

 over is generally far removed from another? The ex- 

 planation might be found simply in the fact that each of 

 the "leptotene" chromosomes— i. e., the finely drawn out 

 chromosomes which are just about to undergo synapsis— 

 pursued a general course that had few close turns in it. 

 (For possibly it maintains the same general direction as 

 it had when it was short and thick ; the reader will recall 

 that Boveri found that chromosomes preserve their ap- 

 proximate shape and position from one cell division to the 

 next.) When, therefore, the leptotene chromosomes are 

 being brought together by the synaptic attraction which 

 homologous loci then bear for each other, the threads are 

 usually crossed only at a few points, and these are gen- 

 erally far apart. If these initial points of crossing— 

 which, it will be observed, have been determined by the 

 original positions of the threads, and not by any twisting 

 —are the points of crossing-over, interference would be 

 accounted for, and would, in effect, be of the same general 

 nature as on the mechanism of crossing-over postulated 

 by Janssens. 



It might at first seem hard to imagine why, on this 

 second scheme of crossing-over, recombination — i. e., 

 11 crossing-over "—should occur where the threads cross, 

 but it should be remembered that the two threads, while 

 coming together, often lie in about the same plane both 

 above and below the point of crossing. If they keep to this 

 original plane as they draw together, they will come to 

 have the same plane of apposition just above and just 

 below the crossing point,— although the sides of the fila- 

 ments that face each other will be just the opposite in the 

 two cases ; consequently, the threads at the crossing point 

 must undergo a very sharp twist, and if, as we must sup- 

 pose, they are somewhat viscous, this may result in their 

 breakage and recombination, or, perhaps, first in their 

 fusion, and, later, when the pieces of the same chromo- 

 some above and below the point of crossing are wrenched 



