30i > 



THE AMEBIC AN NATURALIST 



[Vol. L 



Finally, Janssens says that crossing-over is indicated by 

 the fact that the chromosomes often seem to have sunken 

 into one another at the crossing point. This detail, which 

 would be very difficult to establish, might, of course (if 

 it had any significance at all), merely mean that the chro- 

 mosomes were still closely attached at the point where 

 they had previously crossed-over. It seems incautious, 

 therefore, to regard the cytological evidence as showing 

 more than the possible means of the crossing-over which 

 the evidence from factor and chromosome distribution 

 demonstrates to occur. 



Some of the Orthopteran material which gives such 

 clear-cut chromosome figures might perhaps settle the 

 point whether crossing-over occurs at the stage of four 

 threads, as Janssens believed. For it is reported by 

 Wenrich (19) that homologous chromosomes can some- 

 times be distinguished from one another in prophases by 

 differences in the size or shape of contained granules, that 

 are constant for the particular individual. Moreover, the 

 four threads are clearly distinguishable in the prophase 

 of the first maturation division. If a female could be 

 found (there is some reason to believe that crossing-over 

 does not occur in the male) which showed a difference in 

 respect to two granules at different points in the same pair 

 of chromosomes, then, if Janssens's theory is right, it 

 would happen that, in some of the oocytes, of the four 

 post-synaptic threads two would have a new combination 

 of granules and the other two would not show any inter- 

 change. But on the view that crossing-over occurs earlier 

 — the identical halves being formed after interchange has 

 taken place— all four threads would be of a new combina- 

 tion in those cases where crossing-over of chromosomes in 

 the region between the two pairs of granules had oc- 

 curred at all. 



There is an essentially similar possibility of finding out 

 the same thing genetically. For if two threads may cross 

 over and not the other two, then, if non-disjunction of 

 X-chromosomes should occur in that maturation division 



