7 ///■; Ml TATED OENE 17 



life. Another problem, which is illustrated in the diagram, is 

 whether this period is fixed as a definite stage in development or 

 its place in time is relatively different under different temperature 

 conditions. This special problem will have to be discussed later 

 in connection with the rate concept. In this ••use, the effective 



period falls within the third instar, a time at which the eye 



Aiihni! exists as an imagina] disk; according to Driver, it begins 

 about the time when optic and antenna! ArUagen are separating 

 from each ot her, i.< .. very early in eye development. It ends, as 



expected, when actual facets are being formed. 



31 



30 



29 



J27 



8 22 



& 17 

 w 



5 17 

 a. 

 E 

 <u 



»- 30 

 27 

 22 



17 



Vestigial 



lie terozygoles 



Wild 



50 100 150 200 250 300 350 400 450 500 550 



Hours 



Fig. ."). J):ita for the lengths of the developmental periods and tin- positions 

 and lengths of the temperature-effective period f>>r wing length in Drosophila. 

 The long linos represenl the earlier developmental peri ids. The solid portions 

 indicate the larval periods; the broken ones, the pupal periods. The upper short 

 lines represenl the temperature-effective periods for wing lengths of the females; 

 the lower lines, those of the males. (From Stanley, l'.».'5">. ./. Exp. Zool. 69, 

 Fig. 17.i 



Another case upon which much work has been done is the 

 vestigia] wins. It was reported that the vestigial wing in 

 I >ro8opbila could be induced l>y increase of temperature to change 

 into the phenotype of all the other t^-allelomorphs. Stanley 

 (1935) determined the sensitive period for the production of 

 these phenocopies as well as for corresponding effects upon 



