OF MUTANT CHARACTERS. 199 



locus experiments in the balancing of the black purple vestiKLil back- 

 cross. The inviabiUty of vestigial met with in the early purple vesti- 

 gial crosses seems not to have been due primarily to " prematura! ion," 

 "repugnance," or autosomal lethals, but probably to culture condi- 

 tions, as shown by Carver (unpublished). 



In the development of autosomal linkage, purple was involved in the 

 first coupling F2 back-cross test of crossing-over in the male, and like- 

 wise in the female. One of these back-cross tests of the male gave a 

 few apparent cross-overs which prevented a clear conception of "non- 

 crossing over in the male." The back-cross tests of the female gave 

 the first "two-point" autosomal map, purple vestigml. The first 

 autosomal "three-point" map was black purple vestigial, completed 

 by the determination of the black-purple cross-over value. 



With that most fascinating and difficult subject — the analysis of 

 the relation between the physical chromosome and the process of 

 crossing-over — purple has been intimately connected. The relatively 

 high coincidences obtained in the cases of black purple curved and 

 black purple vestigial soon showed that this relationship in the purple 

 region of the second chromosome is different from the relationship for 

 sections of like map-distance in compared regions of the first chromo- 

 some. An explanation of this comparative study should aid in arriving 

 at the cause of the differences. 



A study in the case of black purple curved of the change in coinci- 

 dence accompanying the age variation in crossing-over (Bridges, 1915) 

 led to the tentative conclusion that both changes were mainly due to 

 a lengthening of the average length of the section of chromosome 

 between simultaneous cross-overs, rather than to a change in the 



S' 

 freedom of crossing-over. Certain other experiments, notably , 



Pr c Sp 



which give information on the age and coincidence changes, have 

 given results that agree better with the first interpretation, though they 

 do not exclude the alternative. The clearest evidence in favor of the 

 internode change is derived from the experiment made by Plough as 

 a control for his temperature-change cultures (& p,. c B. C, 22°, 2-day 

 interval tube-control). In this experiment the curve for varmtion 

 in coincidence was the mirror image of the curve of variation in age. 

 The curve of coincidence corresponding to the curve of temperature 

 variation found by Plough seems to be a straight line cutting through 

 the rises and falls of the temperature curve and independent of them. 

 This suggests that the temperature variation is due to a change in a 

 physiological factor different from that involved in the age variation; 

 and that probably it is due to a modification of the coefficient of cross- 

 ing-over of the chromosome itself. 



