SECT. 4] HEAT-PRODUCTION OF THE EMBRYO 



707 



)(4 days) 46-6 

 ) 18-3 



Heat production 

 (metabolic rate) 



Heat B.S^H. wt. B.&H,&H. 



HeatB.aH.wts. M. 



Calcd. by Le Breton &, 



Schaeffer from B.SiH. 



from which it can be seen that the fall is most pronounced, the 

 metabolic rate being above 15 gm. cal. per gram on the 5th day and 

 below 4 gm. cal. per gram on 

 the I gth day. The significance 

 of the kink in the curve at 

 the gth day is obscure. As 

 the curve in which Murray's 

 weight measurements are used 

 shows it too, it must be due to 

 the heat data. On the whole, 

 a good resemblance can be 

 traced between the heat-pro- 

 duction metabolic rate curve 

 and the gaseous exchange 

 metabolic rate curves shown 

 in Fig. 143, for in both cases 

 the low value characteristic of 

 the end part of incubation 

 is attained about half-way 

 through development or slight- 

 ly before. The question of re- ^^" '^ 

 lating the heat-production to the surface of the chick embryo, which 

 is rather a complicated one, will be left until the section on Energetics. 

 If now, aided by the investigations of Bohr & Hasselbalch, we enquire 

 what are the number of ^ 

 gram calories produced | 

 during each period of % 

 weight doubling, we find | 

 there is first a fall and > 

 then a rise. In Fig. 147, I 

 which has been plotted e 

 from such a calculation, I 

 the times required to | 

 double the weight at dif- ^ 

 ferent ages are given along 

 one ordinate, while the 



Days 



12 13 14 15 16 17 18 19 



Fig. 147. 



gram calories per gram evolved during the periods of doubling are 

 given along the other. This curve does not at all resemble that found 

 by Gayda for the frog, in which there was a continual rise, and from 



