68 



suggestion that the accessory chromo- 

 some is a sex-determiner. Stevens ('05) 

 and Wilson ('05) verified this by 

 showing that in numerous forms there 

 is a sex chromosome, present in all the 

 eggs and in the female-producing 

 sperm, but absent, or represented by 

 a smaller homologue, in the male-pro- 

 ducing sperm. A further step was made 

 when Morgan ('10) showed that the 

 factor for color in the eyes of the 

 fly Drosophila ampelophila follows the 

 distribution of the sex-chromosome al- 

 ready found in the same species by 

 Stevens ('08). Later, on the appearance 

 of a sex-linked wing mutation in Dro- 

 sophila, Morgan ('10 a, '11) was able to 

 make clear a new point. By crossing 

 white eyed, long winged flies to those 

 with red eyes and rudimentary wings 

 (the new sex-linked character) he ob- 

 tained, in Fo, white eyed rudimentary 

 winged flies. This could happen only 

 if 'crossing over' is possible; which 

 means, on the assumption that both of 

 these factors are in the sex-chromo- 

 somes, that an interchange of materials 

 between homologous chromosomes 

 occurs (in the female only, since the 

 male has only one sex-chromosome). 

 A point not noticed at this time came 

 out later in connection with other sex- 

 linked factors in Drosophila (Morgan 

 '11 d). It became evident that some of 

 the sex-linked factors are associated, 

 i.e., that crossing over does not occur 

 freely between some factors, as shown 

 by the fact that the combinations pres- 

 ent in the Fi flies are much more fre- 

 quent in Fo than are new combinations 

 of the same characters. This means, on 

 the chromosome view, that the chro- 

 mosomes, or at least certain segments 

 of them, are more likely to remain 

 intact during reduction than they are 

 to interchange materials.^ On the basis 



1 It is interesting to read, in this connec- 

 tion, Lock's ('06, p. 248-253) discussion of 

 the matter. 



STURTEVANT 



of these facts Morgan ('11 c, '11 d) 

 has made a suggestion as to the phys- 

 ical basis of coupling. He uses Jans- 

 sens' ('09) chiasmatype hypothesis as 

 a mechanism. As he expresses it (Mor- 

 gan '11 c): 



If the materials that represent these 

 factors are contained in the chromo- 

 somes, and if those that "couple" be near 

 together in a linear series, then when the 

 parental pairs (in the heterozygote ) con- 

 jugate like regions will stand opposed. 

 There is good evidence to support the 

 view that during the strepsinema stage 

 homologous chromosomes twist around 

 each other, but when the chromosomes 

 separate (split) the split is in a single 

 plane, as maintained by Janssens. In con- 

 sequence, the original materials will, for 

 short distances, be more likely to fall on 

 the same side of the split, while remoter 

 regions will be as likely to fall on the 

 same side as the last, as on the opposite 

 side. In consequence, we find coupling in 

 certain characters, and little or no evi- 

 dence at all of coupling in other char- 

 acters, the difference depending on the 

 linear distance apart of the chromosomal 

 materials that represent the factors. Such 

 an explanation will account for all the 

 many phenomena that I have observed 

 and will explain equally, I think, the 

 other cases so far described. The results 

 are a simple mechanical result of the loca- 

 tion of the materials in the chromosomes, 

 and of the method of union of homolo- 

 gous chromosomes, and the proportions 

 that result are not so much the expression 

 of a numerical system as of the relative 

 location of the factors in the chromo- 



SCOPE OF THIS INVESTIGATION 



It would seem, if this hypothesis be 

 correct, that the proportion of 'cross- 

 overs' could be used as an index of the 

 distance between any two factors. 

 Then by determining the distances (in 

 the above sense) between A and B and 

 between B and C, one should be able 

 to predict AC. For, if proportion of 



