III. 5 



HEAT 



101 



increasing rate. Lillie and Knowlton have made the same 

 observation for the species investigated by them. 



Tem.fie'ia.Lu.'ie CenlifftacLe 



S3" 22 V 20 19 18 17 16 ir, IV U r if 11 /0 9 S" 1 6 5 * 3' 2 1 



FIG. 52. Curves showing the effect of temperature upon the rate of 

 development of the Frog (Ranafusca). The abscissae give the tempera- 

 ture in degrees Centigrade, the ordinates the days required to reach 

 each of the stages I to IX. I, gastrula ; II, medullary plate ; III, me- 

 dullary folds closed, suckers ; IV, tail-bud ; V, tail and gills ; VI, tail fin ; 

 VII, operculum beginning ; VIII, operculum closing ; IX, rudiments of 

 hind legs. (After 0. Hertwig, 1898.) 



The rate of growth, however, may increase at an increasing 

 (or decrease at a decreasing) rate, as Lillie and Knowlton found 



