No. 614] PHYSIOLOGICAL PROBLEMS 



131 



ignated as Q 1Q . Vt is the velocity of development at 

 any temperature (t), so that Q 10 is the quotient of 7( ^ Q1) 

 and supposedly is a constant. In fact, the Q 10 is not a 

 constant for living phenomena, but usually varies from 

 2 to 3, being greater for the lower temperatures and 

 smaller for the higher ones. Snyder has pointed out in 

 detail that while the temperature coefficient for differ- 

 ences of 10 degrees varies, the variation is not only for 

 physiological actions, but also for many chemical reac- 

 tions; in both cases the variations are in the same direc- 

 tion. He finds that changes in viscosity with changes in 

 temperature follow the same rule. He holds the hypothe- 

 sis that even in the simpler physiological actions we have 

 to deal with at least two distinct chemical actions whose 

 fundamental velocities at any given temperature are 

 different. 



Recently Krogh ('14) has calculated the Q 10 from Q x , 

 Q 2 and the like, at different temperatures for the time 

 from fertilization of the frog's egg to the appearance of 

 the first cleavage plane. He found 53.0 (published as 

 5.3, which appears to be a error) for the interval be- 

 tween 3 and 5 degrees, 4.1 for the interval 5 to 10 

 degrees, 2.0 for the interval 15 to 20 degrees. He raises 

 the question as to the value of such a variable " con- 

 stant." He calls attention to the fact that the velocity 

 curve (the reciprocal of the time-temperature curve) 

 is a straight line within certain limits. This is not 

 the curve for the reciprocal of Van't Hoff's time and 

 temperature formula. The latter law is valuable only as 

 evidence that the life process is a combination of chemical 

 processes. The condition of any environmental factor at 

 which development does not take place, but immediately 

 above which development may be initiated, is called the 

 threshold of development. It is evident that there is a 

 threshold of development for most species as regards 

 temperature, moisture, light, oxygen, quantity and qual- 

 ity of food, and probably other factors. The brief state- 



