992 ENERGETICS AND ENERGY-SOURCES [pt. iii 



combusted, the fat at the late stages being turned into carbohydrate 

 before being burnt. His arguments for this process, however, were 

 not convincing. 



19. A very striking support for the view that a succession of 

 energy sources takes place in the development of the chick is to be 

 found in the analyses of the white yolk which were carried out by 

 Riddle and by Spohn & Riddle. As has already been mentioned 

 (see p. 286), the white yolk, the earliest pabulum of the embryo, is 

 much richer in salts and in protein than the yellow yolk, which forms 

 its later nutriment. These facts fit in exactly with what has already 

 been said about the constitution of the embryo and its sources of 

 energy. It is almost safe to predict that the white yolk will be found 

 to have a higher percentage of total carbohydrate, or perhaps of free 

 sugar, than the yellow yolk. 



20. When the ammonia, urea and uric acid in the hen's egg are 

 estimated throughout incubation, the absolute amounts rise in regular 

 curves corresponding to the growth of the embryo. When further 

 analysis is made, however, it is found that in all cases the amount of 

 these nitrogenous end-products, when related to wet weight, rise up 

 to a certain point, and then remain at a steady level, while, if they are 

 related to dry weight, they rise to a peak from which they fall again 

 in the later stages of development. Thus in each case it could be said 

 that I gm. dry weight of embryo excretes a maximum of nitrogenous 

 waste at a definite point in development. The obvious conclusion is 

 that a peak of maximum protein catabolism exists, coming signifi- 

 cantly at 8'5 days of development, i.e. exactly between the periods 

 which, on the evidence given above, we believe to be associated with 

 the predominant catabolism of carbohydrate and fat respectively. 

 Fig. 261 shows these relationships, plotting the protein combusted 

 in milligrams per cent, of the dry or wet weight of the embryo at 

 the time against the age. In Fig. 262 the curve of Fiske & Boyden is 

 also given. The investigations of Bialascewicz & Mincovna have also 

 made it likely that a similar peak exists in the frog embryo. Fig. 368, 

 constructed from their data, illustrates this strikingly, for in it the 

 quantity of nitrogen in milligrams excreted by one embryo in each 

 24 hours is plotted against the age in hours from fertilisation. An 

 unmistakable peaked curve is seen, and confirmation of the views 

 already expressed is furnished by the curve for combustion of fatty 

 acids, which rises steadily but later than the nitrogen excretion curves. 



