156 POPULAR SCIENCE MONTHLY.
added field of inquiry, worthy of careful consideration, if we are to possess a clear understanding of nature's processes.
Between the animal and the vegetable cell certain sharp lines of dis- tinction are frequently drawn. Physiologists are wont to believe that the processes characteristic of the cells of animal tissues and organs are essentially destructive, i. e., that they are principally katabolic, while in vegetable tissues, on the other hand, constructive processes are very conspicuous. In no way is this better illustrated than in the prevalent opinions regarding the parts played by the two classes of cells in the metabolism of proteid matter. We are accustomed to think that all proteid matter has its primary origin in the synthetical power of the vegetable cell, aided by its contained chlorophyll and the beneficent action of the sun's rays. The animal cell, on the other hand, can merely transform and reconstruct the various proteids furnished by the vegetable world, being without power to manufacture proteid matter dc novo out of the simple groups and radicles which the vegetable cell utilizes so rapidly. In ordinary proteid katabolism, the various nitrog- enous decomposition products are presumably all converted into urea and allied substances adapted for excretion. If, however, there is reversible ferment or enzyme action in the animal body, why may there not also be power to utilize, in some measure at least, the crystalline nitrogenous bases and amido-acids so abundantly formed in trypsin proteolysis, for the construction of fresh proteid matter ? One may well query, considering the vigor of the proteolytic action of the enzymes poured into the alimentary tract, whether all these nitrogenous waste products represent just so much lost energy in their production and a further loss of energy in their immediate excretion from the body. In harmony with the ' luxus consumption ' theory we may assume wisdom and ultimate gain in this speedy decomposition of excessive proteid foods in the alimentary tract, but the argument is not very convincing. Why may not animal cells, or the animal body as a whole, build up proteid matter out of simple nitrogenous compounds analogous to the action of plant cells ? Loewi* has indeed experimented in this direction and states that the biuret-free end-products resulting from the pro- teolysis of ordinary food albumin can be utilized by the animal body for the maintenance of nitrogenous equilibrium, etc., equally well with the common proteid food-stuffs. His conclusions, however, have been called in question by other investigators, notably by Lesserf whose ex- perimental data failed to confirm the above conclusion.
The problem, however, is an exceedingly important one. If the animal body has no power of utilizing the varied nitrogenous com-
- ' Ueber Eiweissynthese im Tbierkorper, 5 Archiv fur exper. Pharmakol. u.
Pathol, Band 48, p. 303.
t ' Ueber Stoffwechselversuehe mit den Endprodukten peptischer und tryptischer Eiweissverdauung,' Zeitsclir. filr Biologic, Band 45, p. 497.