SECT. 2] AND WEIGHT 521 



investigations on the effect of temperature on embryonic growth. 

 Figs. 81 and 82 show some of the results. Fig. 81 includes the data 

 of Krogh; Lillie & Knowlton; and Hertwig on early development 

 in the frog, and Fig. 82 includes the data of Dannevig; Krogh & 

 Johansen; Higurashi & Tauti, and Higurashi & Nakai on develop- 

 ment in various teleost fishes. The regularity with which values are 

 obtained for /x of about 20,000 is remarkable; thus the first cleavage 

 o£Rana eggs has 21,900, the third cleavage to yolk-plug disappearance 

 in Rana has 22,000, the total incubation time oi Hypomesus olidus has 

 23,700 and of Plecoglossus altivelis 23,000. Blunk's figures for the rate 

 of development of Dytiscus marginalis give 19,300 and of Dytiscus 

 semisulcatus 20,000. Ziegelmayer's figures for the rate of develop- 

 ment of Cyclops give 15,700, which is on the low side. Bliss's figures 

 for Drosophila give a very similar result. The fact that the same 

 temperature characteristic holds good for various different periods 

 during one continuous developmental process obviously suggests that 

 the shape of the curve must be much the same at different tempera- 

 tures. The developmental process as a whole is therefore not deformed 

 by change of temperature, but simply lengthened or shortened, its 

 shape, as it were, remaining the same, or, in other words, either only 

 one velocity constant is involved, or else, if more than one, velocity 

 constants of the same temperature characteristic. To what extent 

 this is really the case will appear presently. 



The only work on the temperature characteristic of the develop- 

 ment of the chick is that of Brody & Henderson, who studied the 

 effect of incubation at different temperatures upon the growth-rate 

 of the embryo. Fig. 83 a taken from their paper shows how at any 

 given day during development the weight attained is greater the 

 higher the temperature; thus at the 17th day the average weight 

 at 35-0° C. is 5-510 gm., at 37-3° 12-685 gm. and at 40-5° 20-427 gm. 

 The upper part of the same graph shows the log. weight plotted 

 against the age, as is usual in Brody's method, and the slopes of the 

 resulting series of straight lines give the instantaneous percentage 

 growth-rates, as follows : 



