18 PHYSIOLOGICAL GENETICS 



t^-heterozygotes and also the small wing-lengthening effect 

 upon Wild-type wings. His results are summarized in the 

 graphic representation I Fig. 5). 



[f these data are compared with the tacts known about the 

 development of wing buds, they point out that the important 

 phenocopic effed (30 and 31°) occurs about the time of the 

 third instar when the wing buds are just being formed. This is, 

 indeed, to be expected on the basis of the facts of development of 

 rowings, which will be reported later. There is, in addition, 

 another very early effect visible at lower temperature, an effect 

 that must have occurred at a time before the wing bud became 

 visible in the larva. In addition, there is a later sensitive period 

 for the heterozygote and the wild type. These discrepancies 

 are caused by a fact, unknown to Stanley, viz., that the effect 

 upon the vestigial wing is something different from the effect 

 upon the Wild-type wing and has nothing whatever to do with 

 growth in length. Only the 30 and 31° experiments upon the 

 vg-w'mg pertain to the phenocopic phenomenon and fix their 

 sensitive period. Harnly's (1936) determinations agree rather 

 well with Stanley's. He fixed the beginning of the sensitive 

 period for all the temperatures that have a phenocopic effect 

 (29 to 31°) at 64 to 68 hr. of larval development, which corre- 

 sponds to the molt between the second and third instar. The 

 duration of the period was, however, somewhat different at 

 different temperatures. 



Let us interpret these facts. It is clear that the process con- 

 trolling the abnormal development of the wing buds sets in as 

 soon as their Anlagen are differentiated. From the facts con- 

 cerning the development of these mutant wings we shall later 

 derive the conclusion that the different degrees of wing scalloping 

 are dependent upon the time of visible onset of a degenerative 

 process. The sensitive period described here is, then, the period 

 in which this process may be counteracted, perhaps by changing 

 its relative velocity, the effects of which would be the same as a 

 later onset of the process. At first sight, it might seem to be a 

 contradiction that the sensitive period for the phenocopic action 

 in the Wild type occurs much later; the reason is that in the Wild 

 type, the temperature shock starts a process that otherwise 

 would not occur and the amount of which is determined by its 

 onset within a period in which such a process might still be 



