986 ENERGETICS AND ENERGY-SOURCES [pt. iii 



ergs are used up. But the segmentation period is only a compara- 

 tively short part of the whole period, so that, allowing for this, a 

 value of 39 ergs was obtained, i.e. of the same order as that found 

 by blocking the first cleavage with ultra-violet fight. It is evident, 

 however, that these calculations can tell us nothing at all about the 

 O.E., for the processes of embryonic development are not reversible, 

 and as there is no equilibrium point there can be no sense in deter- 

 mining the amount of energy required to stop the processes from 

 going on. 



7-7. The Sources of the Energy Lost from the Egg 



Although practically nothing is known about the origin of the energy 

 which forms the O.E., a good deal can be said about the origin of the 

 energy which furnishes the Ea., or total catabolism of embryonic life. 

 As we have already seen in the section on the general metabolism of 

 the embryo, there is some reason for supposing that, during pre-natal 

 life, the principal constituents of the embryonic body reach their 

 maxima in a definite succession, i.e. inorganic substances, carbohy- 

 drate, protein, fat. The possible significance of this from a general point 

 of view was there discussed, and comparison was made between these 

 data and the varying intensities of absorption of these important 

 components. But an ontogenetic succession of carbohydrate, protein, 

 and fat makes its appearance not only when these three substances 

 are considered as elements in the architecture of the embryo but also 

 when they are considered as reserves of energy for the Ea,, fuel- 

 sources for purposes of combustion. Considered in this way, it is 

 certain that in the case of the chick, for instance, carbohydrate 

 attains its highest point in the first week, protein in the second and 

 fat in the third. 



For a long time it was generally considered that fatty substances 

 were the sole sources of energy for the developing chick embryo. 

 This view, which was satisfactory only as the roughest of approxima- 

 tions, grew naturally out of the early researches of Pott; Liebermann; 

 and Tangl & von Mituch. Estimations of the fat in the embryo and 

 that in the rest of the egg showed that a good deal had been lost 

 in transit, actually from 2-1 to 2-76 gm. per egg on an average. 

 Calculation demonstrated, as we have already seen, that this figure 

 corresponded satisfactorily with the carbon dioxide evolved through- 

 out incubation, and the heat produced during the same time. For 



