290 THE AMERICAN NATURALIST [Vol.L 



was evidence that the distances between the two points of 

 crossing-over in double cross-overs were variable; but 

 this again corresponded with the fact that the chromo- 

 somes of Batracoseps and other forms, as seen under the 

 microscope, did not always twist in loops of the same 

 length. Furthermore, if it be supposed that in most 

 maturing eggs of the fly the homologous chromosomes 

 twist tightly enough to cross at least once or twice, as is 

 certainly the case in Batracoseps and many other forms, 

 it must be concluded that at not every point of crossing 

 does actual "crossing-over" (recombination of strands) 

 take place, for it was found that nearly half of the factor- 

 groups emerged without having undergone any crossing- 

 over at all. And this, in turn, corresponded with the 

 observations of Janssens and others, which showed that 

 at some at least of the points of crossing of homologous 

 chromosomes, the latter merely untwisted again without 

 having undergone the "chiasmatype" process. Here, 

 then, was a theory of crossing-over that seemed com- 

 plete, so far as connecting the genetic facts with the cyto- 

 logical observations was concerned. 



B. Possible Mechanisms of Crossing-Over 

 There is one very unsatisfying point, however, in this 

 original scheme of crossing-over. That is, it postulates 

 that crossing-over occurs at a comparatively late stage in 

 synapsis, when the strands have become very much shorter 

 and thicker than the long delicate threads which first 

 came into contact with their homologues (see Fig. 6). 

 Now, in crossing-over the chromosomes must come into 

 contact, and break, at precisely homologous points, other- 

 wise factors would be lost or gained by them when cross- 

 ing-over occurs. But presumably the factors are set very 

 close together in the line, judging by the fact that muta- 

 tions in new "loci" (positions in the chromosomes) are 

 still as numerous as ever, and that, if the whole chromo- 

 some is packed with factors as close together as, judging 

 by their linkage relations, they seem to be at certain places 

 in it, it must contain at the very least 200 factors. It is 



