THE MUTATED GENE 75 



details of these calculations, e.g., value of n, point to the assump- 

 tion that the reaction in question, produced by the Dichaete 

 gene, is the decomposition of a thermolabile catalyst. And 

 whereas the effect is bristle or no bristle, there must also be 

 involved a threshold value for bristle- removing power. The 

 Dichaete gene, then, is assumed to be a catalyst which catalyzes 

 a chain of reactions which accelerates the (in itself spontaneous 

 but otherwise infinitesimally slow) decomposition of a thermola- 

 bile bristle-forming catalyst by catalyzing the production of a 

 destructive catalyst. It must finally be assumed that all genes 

 act essentially in this way. Although he comes to just the same 

 conclusions as the aforementioned authors, in this analysis 

 Plunkett has delved further into the details of chemical kinetics 

 than any other investigator and has shown that the rate concept 

 of genie action also stands the test of subtle mathematical treat- 

 ment. It might be added that the kinetic side of this work agrees 

 again most remarkably with the work on the bacteriophage, but 

 the actual embryological processes are not known as in the 

 vestigial case. 



Considerable work has been done with the eye mutants of 

 Drosophila, which lend themselves to quantitative treatment 

 because the number of ommatidia is a suitable numerical char- 

 acter. We have discussed Zeleny's and Krafka's work, which 

 has been carried further by Driver and Hersh, whose experi- 

 ments on temperature action and the sensitive period were 

 reported. In these experiments, the following additional results 

 were found (Driver, 1931). The temperature effect, viz., the 

 increase in facet number per degree of decrease of tempera- 

 ture, follows a geometrical formula which Driver expresses as 

 (1 -f- r n ) = Fi/F 2 , where r equals the rate of change, n the number 

 of degrees decrease in temperature, Fi the facet count at the lower, 

 F 2 that at the higher temperature. This progressive effect 

 shows a definite change in rate at 21° in Bar flies but not in 

 Ultrabar. This action of temperature occurs during the sensitive 

 period; but the length of this period (as percentage of larval life) 

 in this case is not the same at all temperatures, becoming smaller 

 with increase in temperature. Ultrabar, however, does not show 

 this variation but shows a change with the temperature, of the 

 onset of the period relative to developmental stage. In addition, 

 it must be stated that the normal eye shows very little of all 



