The Energy-Sources in Ontogenesis

Abstract

The uric acid content of the hen’s egg has been investigated from the fourth to the twentieth day of incubation. There is a period of intensive uric acid production from the seventh to the eleventh day. After that point the excretion of uric acid fails to keep pace with the growth and differentiation of the embryo. The point of maximum intensity of uric acid production occurs two days later than the point of maximum intensity in the production of urea. From the fourth to the seventh day more urea is present than uric acid, and more is excreted, but by the tenth day the adult relationship is attained, in which 95 per cent. of the total nitrogen excreted is uric acid. The maximum intensity of protein combustion is attained between the eighth and the ninth days. It is pointed out that this occurs midway between the periods when carbohydrate and fat are respectively the predominant energy-sources. The protein used as a source of energy belongs entirely to the coagulable fraction; ovomucoid is not employed for this purpose. The protein nitrogen lost by combustion during development amounts to 7.5 per cent. of the total protein nitrogen present at the beginning, and to 3.0 per cent. of the total foodstuff burnt. The R.Q. for each day of incubation has been calculated on the basis of chemical analyses of fat, protein, and carbohydrate, and agrees as well as can be expected at present with those experimentally determined by Bohr and Hasselbalch, and by Lussanna. Further evidence has been collected from the literature indicating that in embryogenesis there is a succession of sources of energy, carbohydrate preceding protein, and protein preceding fat. Injection experiments and other considerations lead to the conclusion that factors located in the embryo decide what the embryo shall make use of as a source of energy. It does not, for instance, combust protein because its supply of available carbohydrate has been exhausted.