SECT. 4] HEAT-PRODUCTION OF THE EMBRYO 749 



genese" and "thermolyse", a controversy now almost a century old 

 and still vigorously proceeding. The two points on which all the 

 disputants are agreed are ( i ) that animals give off to the calorimeter 

 more calories per kilogram the smaller they are, and (2) that, per 

 square metre of skin surface, they all give off much the same amount 

 of heat. These two generalisations apply certainly to homoiotherms, 

 and probably also to poikilotherms, with certain reservations. But 

 the great divergence of opinion arises when we try to decide whether 

 the surface area is the cause or the effect of the heat loss. Two schools 

 of thought have come into being on the question. For one school 

 the essence of the interpretation is Newton's law of cooling; a given 

 amount pf surface necessitates inexorably a certain loss of heat, which 

 must consequently be supplied by the protoplasm of the body; 

 "thermolyse" is the cause of "thermogenese". The factors which 

 cause this intenser metabolism in the case of the smaller creatures, 

 i.e. those which have most surface in proportion to their weight, are, 

 for the adherents of this view, all of one kind, and involve differences 

 only of degree. These differences are regarded as being due to 

 anatomical factors. "The tissues of the various homoiotherms — very 

 similar in composition", says Terroine, "have a more or a less in- 

 tense metabolism because by the perfectly coherent operation of the 

 circulatory and respiratory apparatus, they receive in unit time 

 variable amounts of food and of oxygen." Obviously on this view 

 all protoplasms are identical, and the protoplasm of the tgg of a 

 mouse could equally well go to form an elephant if it were not for 

 the fact that the eventual form, shape, size and anatomical arrange- 

 ment of the mouse exists in potentia in the egg-cell of the mouse, 

 ensuring that the end-product of development shall be an object 

 like a mouse with a proportionately large surface. The mouse proto- 

 plasm is, as it were, exactly the same as the elephant protoplasm, 

 but destined to work a great deal harder because something in the 

 mouse ^gg arranges that development shall stop when a certain small 

 size is reached, and therefore that the energy turnover in unit time 

 shall be considerable. The eventual surface and the eventual morpho- 

 logy, on this view, are what is originally given in the egg-cell — an 

 almost Aristotelian conception which suspiciously resembles the pro- 

 position "au commencement etait la forme". Perhaps we may see 

 in this attitude another expression of that point of view which has 

 been so ably discussed by E. S. Russell in his book Form and Function. 



