752 THE RESPIRATION AND [pt. iir 



must be admitted that the probabilities are much in favour of 

 LeBreton's interpretation, as far as the embryological evidence is con- 

 cerned. In post-natal life, as we have seen, it has often been assumed 

 that the surface is somehow proportional to the active mass, but in the 

 embr^'o this may not be so. The individual organism, as regards its 

 protoplasmic metabolic intensity, follows the curve of its species, and 

 comes at last to an equilibrium point, at which just enough surface 

 has been developed to carry away conveniently the heat arising from 

 its hereditarily determined protoplasmic metabolic intensity. 



The surface is not the only entity that grows more slowly than the 

 total body-weight. Brody showed that the instantaneous percentage 

 growth-rate of oxygen utilisation and carbon dioxide elimination in 

 the chick embryo is not the same as that of the whole body^. This 

 affords an obvious indication of an active protoplasmic mass growing 

 (like the surface) less rapidly than the embryonic body as a whole. 

 If Brody's instantaneous growth-constants are compared they work 



out as follows : 



Table 87. 



"Either the COa-producing mechanism", says Brody, "develops at 

 a constant percentage rate independent of the increase in body- 

 weight, or the weight of the body or its constituents cannot be taken 

 as an index of the growth of metabolising tissues." These important 

 facts illustrate from an unusual angle the constancy of heat output 

 per unit surface. The surface is not the governing factor, but rather 

 something else growing also more slowly than the total body-weight. 

 There is no need to discuss the attempts which have been 

 made to identify chemically this active protoplasmic mass, or the 

 paraplasmatic proteins, for they do not directly concern the embryo. 



1 See Figures 526 and 536. 



