240 Sturtevant. 



crossing over", on which there are now at hand somewhat more data 

 than before. 



Double Crossing Over. 



The mechanism used by Morgan ('lib, 'lie) to explain crossing 

 over is the chiasmatype described by Jaxssens ('09). During the growth 

 period of the germ-cells homologous chromosomes twist around each 

 other, and then separate again. According to Jaxssexs they do not 

 always simply untwist, but there may be a fusion between them, and 

 when the separation occurs it may happen in the manner shown in 

 figure 2. 



Since numerous twists have been seen in the same pair of chromo- 

 somes it would seem likely that more than one such interchange may 

 occur between them. If this occurs it should lead to some interesting 



Fig. 2. 



results in the distribution of genes. That it really does occur has been 

 shown l)y making crosses involving more than two sex-linked pairs of 

 genes. Such crosses were first reported by Morgan ('lib). Since 

 then several others have been recorded, and four sets of them were 

 analyzed briefly in my former paper. The accompanying table (III) 

 shows the results obtained from all such crosses involving chromosome I 

 which are now available'). As in the case of table I, the detailed data 

 will be found at the end of this paper (table XVI). 



Some of these crosses were made primarily for other purposes, 

 and in several of them the counts are obviously too small to be of great 

 significance. As a whole, however, they furuish perhaps the strongest 

 evidence yet produced iu favor of the chromosome explanation of linkage. 



They give a complete confirmation of the relative positions assigned 

 to the various genes in the last section; for, in each experiment that 

 ai'rangement gives a simple explanation of the smallness of two com- 



'■) The case of WVM (Morgan '11) and a few counts in the case of YWM 

 have been omitted because of obvious discrepancies. Note of these cases is made in 

 table III. 



