618 JOHN R. MUSLIN 



which could be ascribed to any developmental process as opposed to the 

 maintenance processes would be very .small. 



Rubner(?rt) believes that the law of skin area is applicable to the em- 

 bryo. He calculated that the metabolism of the new-born mammal, assum- 

 ing its weight to be 8 per cent of that of its mother, would be nearly double 

 as much per kilogram and hour as that of the mother. 



Because the embryo is less active in every way than the new-born its 

 metabolism per unit of weight should be considerably less than this, which 

 indeed the results of Bohr and Tangl show to be the case. Rubner ex- 

 plains the higher metabolism of the embryo per unit of weight, therefore, 

 as due not to any specific requirement for developmental energy, but en- 

 tirely to the greater loss of heat by the relatively greater surface. He is 

 obliged, however, to eliminate the first four-tenths of the embryonic life 

 from the operation of this law, because within that period the embryo is. 

 of no appreciable size as compared with the mother. On the basis of the 

 average composition of living substance in mammals and using seven 

 tenths of the metabolism of the new-born as the rate for the embryo, 

 Rubner calculates that for the remaining six-tenths of the gestation period 

 the "growth quota" of the embryo in most mammals is from 38 to 40 

 per cent of the energy supplied, as compared with 34 per cent for extra- 

 utciiue life. In other words, for each calorie of heat value stored in the 

 new-born nearly two calories of energy must be expended, while for each 

 calorie deposited in the embryo only one and one-half calories need be 

 expended (on the basis of 40 per cent). We shall see that the higher 

 metabolism of the embryo and fetus is continuous with that of the new- 

 born. 



The qualitative differences in the metabolism of the embryo from that 

 of the adult depend on the kind of food material supplied by the mother 

 in the egg (oviparous development) or by the circulation (viviparous) 

 for the nutrition of the embryo. A hen's egg contains no carbohydrate; 

 hence the respiratory quotient in development of the chick can never be 

 greater than 0.78 (see page 560). The chemical studies of Liebermann, 

 the calorimetric determinations of the heat of combustion by Tangl and 

 the metabolism studies (using the direct and indirect methods) by Bohr 

 and Hasselbalch all agree in showing that the material oxidized in the 

 development of the chick is fat. Liebermann believed that some nitrogen 

 was lost, but both Hasselbalch and Tangl and Mituch have shown that 

 this is incorrect. The nitrogenous building material is not used as a source 

 of energy. 



The source of energy for the silkworm embryo, according to the chem- 

 ical studies of Tichomiroff and the respiration experiments of Farkas; 

 for the blow-fly embryo according to the respiration experiments of Wein- 

 land ; and for the cadaver fly according to the calorimetric determination 

 of Tangl is likewise mainly fat. Xo nitrogen is lost in gaseous form dur- 



