JOSEPH KRAFKA, JR. 453 



As shown by the constant results under constant external con- 

 ditions, the hereditary mechanism remains the same. The reactions 

 involved in differentiating the somatic tissues have been shown to 

 be modifiable through various external factors, chief of which is 

 temperature. 



Consideration of the Means hy Which Temperature Can Produce an 

 Effect on Facet Number. 



We have seen in the foregoing pages that facet number in the 

 full-eyed wild fly is afifected very little by temperature, while in the 

 bar-eyed mutant there is a very marked effect. It is also evident 

 that the effect is produced only through a relatively short period in 

 larval life. What hypothesis can explain these facts? 



Assuming a normal mechanism for facet production (A, B, C, D, 

 E), we may say that temperature affects the rate of all the various 

 processes involved in nearly equal amounts. The rates are increased 

 at the higher temperatures, but at the same time the length of time 

 of the reactions is proportionally shortened, thereby producing a 

 constant number of facets (N) in the full eye. 



In the bar eye a new condition has come about. The facet num- 

 ber is reduced to about t that of the full eye at 15° and to about ji 

 at 27° in the Low Selected line. In Ultra-bar the reduction is even 

 greater; 14 at 15° and 55 at 27°. 



/N\ 



Hypothesis 1. Reduction in Facet Number I — 1 Due to a Reduction 



{n\ . ^^^ 



in the Facet-Forming Substances I ~ )• — As in the case of the full eye, 



increased rates at the higher temperatures with a decrease in the time 

 of the reaction would produce a constant number of facets from a 

 given amount of material at all temperatures. 



Hypothesis 2. Surface Tension. — The number of facets in the bar- 

 eyed stocks varies inversely with the temperature. The reaction 

 thus has a negative temperature coefl&cient suggesting physical phe- 

 nomena and particularly surface tension. The values of Qio are far 

 too high even to approximate the coefficients of any of the possible 

 physical phenomena. Furthermore, if surface tension were the factor 

 involved, we should expect it active in the full eye also, there pro- 

 ducing similar temperature effects as in the bar. 



