SECT. 4] HEAT-PRODUCTION OF THE EMBRYO 747 



ment of the mould Sterigmatocystis nigra which closely resembled that 

 of a growing population of cells such as the chick embryo. Among 

 the most important of the suggestions that have been made with 

 regard to it is the notion of the "masse protoplasmatique active", 

 in contrast to the "ballast", which was introduced by Friedenthal, 

 and has since been much developed by the Strasburg school (Faculty 

 of Medicine). As differentiation goes on, and the make-up of 

 the embryo becomes ever more and more complicated, there must 

 be a constant increase in the amount of storage substances which 

 have themselves no respiratory function, but which participate in 

 the total weight of the embryo. These substances, which are known 

 as "substances paraplasmatiques", will, if they increase out of pro- 

 portion to the size of the growing embryo, obviously have the effect 

 of lowering its unit respiratory activity. Cohn & Murray suggest 

 that the lipoid granules of the central nervous system and the brown 

 pigment of the heart and liver may be substances of this class. 

 Kassowitz and Miihlmann have discussed the accumulation of 

 "metaplastic" bodies in cells during the growing process. 



Another and most important factor which has to be taken into 

 consideration in this connection is the relation between the surface 

 and the volume. As Cohn & Murray put it, " It is in the very nature 

 of geometric relations that with growth the \'olume or mass increases 

 as the cube, and the surface as the square of a number. The result 

 from a biological standpoint is that for a unit mass of active proto- 

 plasm undergoing continuous chemical changes, the portals, that is 

 to say, the surfaces of the organism for entry and exit of the sub- 

 stances which are the antecedents or products of vital activity be- 

 come continuously smaller, and therefore continuously less suitable 

 for maintaining the original velocity of metabolism. There must 

 necessarily follow a diminution of activity and all the other changes 

 that are merely the logical outcome of the initial modification." 



The surface/volume theory must, however, be used with care, for 

 it has various implications other than those which appear at first 

 sight. If the embryo maintained a perfectly spherical shape as it 

 grew, then the theory could be applied to it in its simplest form, but 

 the active surfaces are so numerous and so large that a great number 

 of complicating factors must, at any rate, be admitted into the dis- 

 cussion. Thus in the case of the chick embryo, its effective surface 

 is not only its skin, but also a collection of structures such as the 



