686 



THE RESPIRATION AND 



[PT. Ill 



also necessary to remember that the unabsorbed yolk-mass will in the 

 early stages be included in the weight estimations, though it cannot 

 be counted as thermogenetic tissue. It is easy to understand the 

 decline in metabolic rate with advancing age, for the surface, i.e. 

 the means of exit from and entrance to the body, does not grow as 

 fast as the weight, but it is difficult to understand, on the view held 

 by some physiologists that thermolysis is the cause of thermogenesis, 

 how the embryo can have an increasing metabolic rate in the early 

 stages — as it assuredly does — when every moment the surface/volume 

 ratio is diminishing. The factors controlling the production of heat 

 must be sought somewhere within the body rather than at the surface. 



Gayda also discussed the interesting relations that exist between 

 the heat-production and the time required to double the weight. 

 Curves for these values are 

 shown in Fig. 135. The S-shaped t 

 nature of the curve is striking, ^ 

 but perhaps the lowering at the f 

 older stages is brought about by I 

 metamorphosis, and so lies out | 

 of the strict part of this discus- .| 

 sion. During the greater part of ^ 

 the larval period, both before ^ 

 and after feeding has com- 5 

 menced, however, the parallel- J 

 ism between the two curves is 

 close. Obviously, the less heat 

 that is evolved during the doubling of the weight, the more efficient 

 will be the embryo or larva, and the more economically the turnover 

 will be progressing. Fig. 1 35 shows that the least heat is evolved in the 

 earliest stages, i.e. shortly after hatching, so it must be concluded that 

 the greatest efficiency exists then. The most inefficient point would 

 appear to be at the weight of 40 mgm. Whether it is significant that 

 just at this point feeding begins is not clear. These relations are the 

 direct opposite of what has been found to hold in the case of the 

 chick, which at the 3rd day of incubation appears to be very in- 

 efficient, but which attains a maximum efficiency a few days before 

 hatching. A full treatment of this point is given in Section 7-5. 



Gayda also calculated the temperature coefficient of the heat pro- 

 duction, and found that it worked out at an average of about 2- 10, 



10 20 30 40 50 



Wfc. of embryo or larva in mgms. 



Fig. 135- 



