20 



PHYSIOLOGICAL GENETICS 



not less than 3.8 and not more than 7.") per cent of the egg-larval 

 period. The sensitive period for any one fly lies between the 

 time when 89.3 per cenl and the time when 96.8 per cent of the 

 egg-larval period have been completed. We shall meet these 

 tacts again in later chapters. 



(In a recent paper, [ves (1935) criticizes the results of Child, 

 because ocellar bristles may be influenced at all times by a 

 temperature of 40 to 41°. But so high a temperature may act 

 very differently, by actual destruction of material.) 



60 



72 



84 



24 36 48 



Pupal age in hours 



Fig. 6. — Curves for the breadth of the area enclosed by the symmetrical bands 

 in the wing of Ephestia kuehniella after temperature action at different stages. 

 The horizontal lines represent the controls. {From Kuehn and Henke, 1936, 

 Ab. Ges. Wiss. Goettingen, Fig. 93.) 



Finally, one case has to be reported which is of special sig- 

 nificance because it will furnish us later with important informa- 

 tion regarding the meaning of the phenomenon. In the extended 

 studies upon the wing pattern of the flour moth Ephestia kuehni- 

 ella (Zeller), Kuehn and Henke and their students furnished 

 many facts pertaining to this problem. They found (Kuehn and 

 Henke, 1936) mutants that produce a shift of the two systems 

 of symmetrical bands on the wings toward its center, thus nar- 

 rowing the central field, called the field of symmetry (see Fig. 7). 

 The same effect may be produced as a phcnocopy by treating the 

 pupae at a definite stage with heat shocks (45° for 45 min.), and 

 the sensitive period for this part of the wing pattern was fixed 

 at a time 48 to 60 hr. after pupation at 18°. Figure 6 represents 



