748 THE RESPIRATION AND [pt. iii 



membranes of the allantoic and amniotic sacs, the blastoderm 

 covering the vitelline membrane, and the renal tubules and glomeruli. 

 As the chick is a metazoan animal, there are also the individual 

 surfaces of the cells to be considered. Murray has summarised as 

 follows a number of the points which have to be borne in mind in 

 considering this question, "(i) In the case of individual cells of 

 metazoa which are, as far as we know, of about the same size through- 

 out life, the average of their surface/volume ratios would not change 

 with development any more than it would in a growing colony of 

 unicellular organisms. (2) The surface/volume ratio may theo- 

 retically be maintained at any level simply by the infolding or 

 wrinkling of the surface, as is seen in the intestines. (3) In actuality 

 the area of capillary surface is adjustable since the development of 

 new vessels such as is seen in the processes of repair may occur as 

 the result of the repeated vigorous functioning of a part. (4) It is 

 known that under normal conditions only a fraction of the capillaries 

 and therefore of an exposed surface is open or active at any one time. 

 For instance the amount of heat-radiation from the skin actually 

 depends less upon the measured skin surface than upon vascular 

 changes, which, in turn, depend upon the metabohc rate rather than 

 vice versa. (5) It is not only the area of the surface but the permeability 

 of a surface that is important, and as the chemical constitution of 

 each cell changes markedly with age, so will the surface permeability 

 change. (6) The hypothesis that growth is correlated with the area 

 of absorptive surface supposes that through a given unit of surface 

 a certain restricted number of molecules may pass in unit time. But 

 if, as we know, there is change with age in the kind and therefore 

 the size and migration-rate of molecules which enter the cell, one 

 would hardly expect this simple relationship to be maintained. 

 (7) Growth or storage is the difference between absorption and 

 elimination. Either one of these factors may vary more or less inde- 

 pendently of the other and thus growth is necessarily dependent on 

 both of them. As the ratio of storage to elimination changes with 

 age, if absorption is dependent upon surface, growth cannot be, and 

 vice versa'' It is nevertheless much to be wished that accurate measure- 

 ments were available of the extent of the active surfaces in the chick 

 embryo at all stages of its development. 



The whole question is, of course, bound up with the controversy 

 on the relation between heat-production and heat dispersal, " thermo- 



