THE MUTATED GENE 167 



as steps in a chain reaction in the order ca + — + v + — > cn + . ca + is 

 assumed to be formed first because it is present in full concentra- 

 tion in flies lacking v + , cn + , or both. 1 It reaches effective con- 

 centration before puparium formation; v + , however, only later. 

 V + is supposed to be formed prior to cn + because (among other 

 facts) a y-implant, deficient in cn + -substance if grown in a 

 cn-host (with the same deficiency) gives Wild eye, thus proving 

 its ability to produce cn + . In the production of these three 

 substances, a large number of genes is concerned. For the first 

 step no other gene but ca could be found to be effective. The 

 facts mentioned above for the transplants of many mutant disks 

 into v-hosts show that a number of these genes are concerned with 

 the production of at least the v+-substance, e.g., pn, pr, sf, etc., 

 partly in the eye, partly in the body, e.g., v, p, rb. Cases in 

 which cn-implants are only partially affected by the mutant host 

 indicate genes concerned with cn + -production ; bri and mah are 

 mentioned as such. It is clear that thus far these relations 

 cannot be expressed in concrete terms. A general description 

 would probably again assume the form of the theory of reactions 

 in tune. 



In other chapters, we reported the considerable amount of 

 quantitative work done with the Bar locus in Drosophila. 

 Hersh (1929), who pushed the analysis furthest, has also used 

 this material for an attack upon the problems of this chapter. 

 He took a selected homozygous stock of Bar, supposed to be 

 freed of heterozygous modifiers, as a basis and studied the effect 

 of other mutant genes in the X-chromosome upon facet number. 

 For obvious reasons (one X), the males only were used for the 

 counts. Hersh took into account also another set of facts, to be 

 reported below (page 215), viz., the different growth rate of the 

 dorsal and ventral eye lobes, which follow the equation for 

 heterogenic growth (see page 216) : Y = bX k , in which X and Y 

 are the facet numbers in the dorsal and ventral lobes; b the size 

 ratio of the lobes at the time of facet formation, i.e., the initial 

 difference; and k the ratio of the logarithmic rates of facet forma- 

 tion. The results of this inquiry are summarized (in simplified 

 form as compared with Hersh's table, 1929) in Table 20, which 

 compares facet numbers of a forked Bar line with those after 



1 Recent work shows ca + and v + to be identical. The seat of production 

 of the substances is eye, fat body, Malpighiun tubules. 



