SECT. 4] HEAT-PRODUCTION OF THE EMBRYO 



685 



Gayda, attempting to explain the slow fall in heat-production and 

 metabolic rate after the 20th day, discussed the relations between 

 surface and volume. As the larva grows in volume so its surface 

 must proportionately diminish. What is so striking about the peaked 

 curve for metabolic rate is that in the very earliest stages of develop- 

 ment, while segmentation and gastrulation are proceeding, the heat- 

 production per gram per hour is increasing in spite of the fact that 

 every moment the surface is diminishing in proportion to the volume 

 and the weight. After the main inflection in the curve a simple 

 surface heat-radiated relation is conceivable, but not before it. It is 

 probable, of course, that the dermal surface is not the active surface, 

 or rather not completely coter- 

 minous with it. An immense 

 field of study exists in the deter- 

 mination of the surfaces in the 

 growing embryo and the identi- 

 fication of the active one. Gayda 

 found that, after the 20th day, 

 if the gram calories produced 

 were related to the surface of 

 the embryo (calculated by the 

 VW^ formula) the result was 

 almost a constant, though at 

 first there was some divergence. 

 Thus about the 25th day 100 

 sq. mm. radiated 0-211 gm. cal. per hour, but on the 97th day 

 0-171 gm. cal., and on the 131st day 0-169 gm. cal. In fact, the 

 gram calories liberated per square milHmetre per hour form a curve 

 which declines from the 25th day; this is represented in Fig. 134. 

 If it is compared with Fig. 166 a, in which the calories radiated from 

 I sq. metre per hour are plotted against the age in the case of the pig 

 and the human being, the resemblance is very striking. I shall return 

 to this point. Gayda himself did not see anything important in this 

 peak, however, and considered that it was probably due, on the one 

 hand, to the change in shape of the embryo from spherical to axial, 

 and, on the other hand, to the first swimming movements of the 

 embryo about the time of hatching, believing that, if a constant could 

 be introduced into the calculations to allow for such changes, the peak 

 would entirely disappear. This may or may not be the case, and it is 



Days from fertilisation 

 Fig. 134. 



