1. The total carbohydrate content of the trout egg and embryo has been measured. The carbohydrate content falls during three periods: (i) the establishment of the blood circulation; (ii) hatching; and (iii) the onset of starvation.

2. The fats extracted by a non-polar solvent (carbon tetrachloride) from egg, embryo and yolk samples show no significant catabolism before hatching. These fats are considered to be glyceride-fats. In the yolk-sac stage such fats are only consumed between the 63rd and 80th days of incubation at 10°C.

3. The combustion of phosphatide fat deduced from the heat-production figures is conspicuous during the early stages of yolk-sac absorption, and may be correlated with the consumption of protein as an energy source.

4. These findings may be correlated with histological changes in the yolk-sac wall, and in the relation between yolk-sac and liver. The concept of a sequence of energy sources in ontogenesis is discussed.

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