SECTION 7 



THE ENERGETICS AND ENERGY-SOURCES 

 OF EMBRYONIC DEVELOPMENT^ 



7*1. The Energy Lost from the Egg During Development 



Many investigators, realising that, owing to the changing composi- 

 tion of the embryonic tissues and the raw material of development, it is 

 difficult to compare different entities if the material exchange is alone 

 considered, have thought it worth while to investigate the energetics 

 of the transformations in question. Sometimes this has been done, 

 as we have already seen, by measuring the heat produced during a 

 given developmental period by the embryonic tissues, sometimes it 

 has been done by combusting the embryo and the yolk in the bomb 

 calorimeter, and obtaining in this way data for the amount of energy 

 stored in the substance under investigation. It is with researches of 

 the latter type that this chapter will largely be concerned. 



To establish first the fact that the calorific value of an equal amount 

 of a definite substance may change during the developmental period, 

 it is simply necessary to refer to the figures of Murray, who in 1926 

 estimated the fuel value of i gram of dry substance in the chick 

 embryo between the 5th and the 21st days of incubation. The curve 

 he obtained is shown in Fig. 256, which clearly shows that it rises 

 in a sigmoid form, the points agreeing well enough with the earUer 

 values of Tangl. The increase in calorific value is no doubt due to the 

 decrease of the inorganic and the increase of the organic quota in the 

 embryo. From Fig. 256 it can be seen that the calorific value of i gm. 

 of dry embryo at the end of incubation is about 6-2, having risen 

 from 5-1 at the 5th day, and Murray pointed out that it was rising 

 up towards the level of the calorific value of the unincubated yolk 

 and white taken together, i.e. 6-94. The variation in fuel value here 

 shown reveals well the drawbacks of the plastic efficiency coefficient. 

 As a measure of efficiency it does not take account of the fact that the 

 units on which it is based are constantly changing in calorific quality. 

 It is interesting that Murray found a divergence between the 

 observed and calculated calorific value in the first half of incubation. 



^ Throughout this book "calorie" means gram-calorie and "Calorie" kilo-calorie. 



