THE Mi TATED 01 VE 19 



initiated before the wing structure is completely determined. 

 In the reciprocal effect, however, a process thai begins early in 

 development i- only -lowed down. The sensitive period then 

 covers such a time in development at which the process (early 

 degeneration caused by the vg-gene) has not yet gone so far that 

 it could not be Blowed dow ii more or less. The reciprocal produc- 

 tion of the same phenocopies, at least in part, from the Wild type 



as well as from the mutant, is then actually the effect of two 



completely different inductions, viz., in • case, the induction 



of a process at a definite time and, in the other, the change of rate 

 of an already existing process; both are hound to lead to t he same 



phenotype, because this is determined, given the same process 



of degeneration, by the product of its rate of progress and the 



time of onset. 



Still more exact data are available for some of the bristle 

 mutations in Drosophila. Plunkett (1926) made such deter- 

 minations for the mutant Dichaete, and his student Child (1936) 

 for scute. These experiments do not exactly belong to this 

 chapter, as the effect of temperature here is not the production of 

 phenocopies. The scute mutant upon which the experiments 

 were performed makes certain bristles on thorax and scutellum 

 disappear. But the amount of suppression of development (or 

 destruction?) of these bristles varies considerably: a certain per- 

 centage of individuals shows the absence of one bristle and 

 correspondingly of all the bristles affected. This percentage is 

 influenced by temperature, and there is a temperature-effective 

 or sensitive period involved here. As the same effect may be 

 produced also by genetic modifiers, this case is at best closely 

 related to the case of phenocopies, and the underlying laws are 

 assumed to be the same. Child (1935) found that the tempera- 

 ture effects are more or less specific for each bristle but that the 

 sensitive period is the same for all, viz., situated at a time interval 

 from 75 to 98 per cent of the egg-larval stage. We recall that 

 in our phenocopic experiments the sensitive period tor bristles 

 in general (all effect- upon bristles) extended also into the pupal 

 stage. Plunkett also found such for the variation of the I lichaete 

 type. By still more refined experimentation and statistical 



treatment of his data, Child was able to calculate the sensitive 



period for the individual fly— as opposed to the determination for 



a population with the following result: This period occupies 



