10 



PHYSIOLOGICAL GENETICS 



Table 3 



It is important to note that Friescn (1935), in producing; 

 phenocopies by X rays, found, as far as I can see, the same sensi- 

 tive periods. Thus he remarks 

 that the period for spread wings 

 occupies the last larval and first 

 pupal day, whereas scalloping 

 is produced one day earlier. It 

 might be added that very exact 

 determinations of the sensitive 

 period are difficult in this case 

 because the lethality after treat- 

 ment is so high that mass cultures 

 with a considerable variation in 

 time of development must be 

 5 used. 



Far more exact determination 

 of the sensitive period is possible 

 in the cases of Bar and vestigial, 

 where the described effects are ob- 

 tained within a normal or almost 

 normal range of temperature. A 

 very considerable amount of work 

 has been done by Zeleny and his 

 school to locate exactly the sensi- 

 tive period for the temperature effect upon the Bar-eye series in 

 Drosophila. Figure 4 represents Driver's (1931) results for Bar 

 and Ultrabar. 



In this connection, we are interested only in the fact that sensi- 

 tive periods exist and that they occupy a definite length in larval 



600 



540 



480 



i?420 

 o 

 -^360 



' Q.300 

 "t 



1240 

 c 



i- 180 

 o 



^ 120 



60 







30 27 2220 1715 



Temperature in degrees C. 



Fig. 4. — The main lines represent 

 the lengths of larval life at the con- 

 stant temperatures for Bar and 

 Ultrabar in Drosophila. The short 

 line to the left of each represents 

 the relative length and position of 

 the temperature-effective period for 

 the females; that to the right of 

 each, the effective period of the 

 males. (From Driver, 1931, J. Exp. 

 Zool. 59, Fig. 7.) 



