No. 593] THE MECHANISM OF CROSSING-OVER 



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difficult to conceive liow this cleavage of ultramicroscopic 

 nicety can take place properly at a stage when the chro- 

 mosomes are so coarse and short. The observations of 

 Vejdovsky and others, taken in connection with the ge- 

 netic results from Drosophila, render it practically certain 

 that the factors are really disposed in an extremely fine, 



long thread or "chromonema," which, during the meta- 

 phase and anaphase of mitosis, is coiled up very closely 

 in more or less spiral fashion (probably within a viscous 

 sheath of some sort), to form the thick dense chromo- 

 somes, but which, in the resting period and during the 

 early stages of synapsis, becomes, to some extent at least, 

 uncoiled and drawn out again. In this state, then, the 

 chromosomes first pair, as shown in Fig. 6.. Thus pre- 

 cisely homologous parts of the frail threads rrfay become 

 apposed to each other, so that this stage, which is called 

 the "amphitene" stage, would seem to be the one best 

 " adapted" for the occurrence of crossing-over. Later, 

 when each chromosome becomes, presumably, a thick 

 spiral, there would seem to be much greater mechanical 

 difficulties in the way of exact apposition and breakage of 

 parts. 



On any possible theory of crossing-over, however, the 

 known facts concerning interference should be capable of 



