A. H. Trow 2S3 



(4) that, the percentage of cross-overs obviously! depends upon the 

 distance apart of the factors concerned, and therefore, conversely, 



(5) that the percentage of cross-overs enables one to allocate the 

 factors concerned to their proper relative positions. 



Let us consider whether the authors' standpoint is a safe one. We 

 may admit the validity of the contention that the chr<iniosonies have 

 an individuality of their own, and that the relatiims of the chronionieres 

 to eacli other remain approximately constant — to do otherwise would 

 render all further discussion futile. 



We may aduiit too that in Drosophila ci-ossed chromosomes probably 

 occur in meiosis. It should be added that the crossing over hypothesis 

 renders the detailed cytological study of these cross-overs in this 

 organism a matter of pressing importance. 



Now, do the homologous chromosomes cross according to the laws 

 of chance, anyhow, anywhere, or according to some geometrical scheme ? 

 It is exceedingly difficult to discover whether the authors have really 

 adequately considered this point. 



Let us suppose that the cross-over takes place once only, and at the 

 nn'ddle point of the chromosome, and let the number of factors (or 

 chromomeres) be reduced to six, and lettered ABCDEF and abcdef, 

 the chromosome being regarded as a paired chromos<jme of a nucleus 

 in meiosis of some F^ plant. We should get the following arrange- 

 ment : 



A B C d e f 



3C 



Crossing over takes place so as to produce the cross-overs 

 Ad, Ae, Af; Bd, Be, Bf; C'd, Ce, Of 

 aD, uE, aF; ID, hE, hF; cD, cE, cF 



in equal nuiidiers, and the non-cross-overs 



AB, AC, BG; ah, ac, be 

 DE, DF, EF; de, df, ef 

 in eipial numbers. 



