moiety.” The hypothesis that the metabolic capacity of the body is limited to analytical processes justified the supposition that, when more fat is laid on than the food contains, the balance comes from proteid substances, which split into nitrogenous and fatty moieties. It has been shown, however, that an animal during fattening may put on more fat than is contained as such in the food, or obtainable from its diet, even though all the atoms of carbon and hydrogen in its proteid food were devoted to its formation. The balance must come from carbohydrates. Perhaps a still more striking illustration of constructive capacity is the power of making glycerin. If a dog receive fatty acids in its diet, it accumulates normal fats. The glycerin which, united with fatty acids, constitutes the fat, was not contained in its food. Starch and sugar are sources of fat. As yet there is no evidence that fat can be converted into sugar. The chemistry of the nitrogen-containing compounds appears to present more difficult problems. Plants build up proteins. Is the animal’s relation to these substances limited to their disintegration? Do proteins inevitably descend from step to step until they reach urea? There are reasons for thinking that, even when dealing with nitrogenous substances, the metabolic power of the body is not exclusively analytical. The liver can make urea from ammonia-salts, such as lactate, or even carbonate, of ammonia—substances more stable, and therefore in the chemical sense simpler, than urea. This is an indication, though a faint one, that the body has a constructive capacity, a power of producing more complex from simpler substances, even in the case of nitrogenous compounds. Beef-tea, mutton broth, meat-extracts have long been regarded as foods of value when the power of assimilation is low. Chemists point out that the nitrogenous substances which these decoctions contain are so near the bottom of the ladder that the energy set free by their further oxidation to urea is scarcely worth consideration. They admit that their ready availability renders them useful as restoratives, but they deny them the status of foods, on the assumption that their further progress must be downward. As was stated when the conversion of peptones into leucin and tyrosin was described, evidence is beginning to accumulate which shows that within certain limits, at present impossible to define, the system can reconstruct its proteins from amides and other simple products of their degradation.