ON THE EFFECT OF TEMPERATURE ON THE 

 DEVELOPMENT OF ANIMALS. 



FRANK R. LILLIE AND F. P. KNOWLTON. 



PROTOPLASM exercises its functions only within a certain 

 range of temperature ; the range extends from a minimum 

 temperature where its functions may be said to begin 1 to a 

 maximum above which occurs heat rigor and death, through 

 an optimum which lies nearer the maximum than the mini- 

 mum. 2 The intensity of any function increases from the 

 minimum to the optimum temperature and then, as a rule, 

 decreases to the maximum, though never to the level of the 

 minimum. This temperature influence is especially well 

 marked in the case of the formative functions of protoplasm, 

 and although the number of authors who have noted this is 

 very great, and although use is often made of it to facilitate 

 observation, yet we have no adequate quantitative study of its 

 effect in animals. The influence of temperature on growth in 

 plants has, on the other hand, been carefully determined. 



The effect of temperature on development in animals is two- 

 fold, - - on the rate and on i\\eform. The latter effect is pro- 

 duced only by temperatures above the maximum or below the 

 minimum. In this sense, then, the minimum temperature for the 

 whole development is the lowest and the maximum the highest at 

 which normal development may occur. The terms maximum and 

 minimum will be used in this sense throughout this paper. 



The effect of temperature on the rate of development in- 

 volves two variants, time and extent, which are directly pro- 

 portional to each other. By making either of these a fixed 

 quantity we can measure the effect of the different tempera- 

 tures in terms of the other. It is generally advisable to fix 

 the extent and to measure the times at different temperatures, 

 because it is not possible, as a rule, to express the differing 



1 We do not mean to imply that there is absolutely no metabolism at sub- 

 minimal temperatures. 



2 See Davenport, Experimental Morphology, vol. i, p. 227. 





