No. 614] 



PHYSIOLOGICAL PROBLEMS 



135 



and 21 degrees, while the limit of development is from 

 less than 3 to 22.1. 



Thus comparing the velocity curves for Headlee's de- 

 velopment of Toxoptera (Fig. 1) and for the cleavage of 

 the frog's egg, we note that in the case of the frog's egg 

 the velocity is too great at the lower temperatures and 

 falls off at the highest temperature. Also, in the case of 

 Headlee's curve for Toxoptera, the development was 

 much too slow at the higher temperatures. Krogh ( '14) 

 further studied the development of pupae of Tenebrio 

 molitor, carefully measuring the carbon dioxide given off. 

 He found that the curve of velocity was a straight line 

 between 18.5 and 28 degrees, but that it curved upward at 

 lower temperatures. He tried incubating the pupee at 

 13.45 degrees, which is the mathematical zero of his curve, 

 and found that they developed in 1,116 hours, but with 

 considerable mortality. At approximately 33 degrees 

 the velocity was less than it should be if the curve were a 

 true hyperbola. An interesting feature of these curves 

 is that they approach so nearly to the curve published by 

 Verworn showing the stimulation effect of heat on activ- 

 ity. This curve is shown in Fig. 2 by the actual velocity 

 curve and the dotted extensions which, when compared 

 with the curve of Krogh for the development of Strongy- 

 locentrotus, Arbacia, and Tenebrio, indicate the close re- 

 lation between the amount and rate of activity and that 

 of general metabolism and growth. 



Edwards ('02) made a careful study of the hen's egg 

 and established 20-21° C. as the point at which no devel- 

 opment takes place. There is an optimum temperature 

 and development is accelerated by slightly higher tem- 

 peratures and retarded by lower temperatures. Thus 

 even in a warm-blooded species there is a point at and 

 below which development does not occur. 



(b) Prediction on the Basis of Temperature Laics 

 Can we predict the time of appearance of any stage in 

 the life cycle of an animal? Certainly, in so far as we 



