[14] Euler, H., _Pflanzenchemie_, 1909, ii. and iii., 140. We may now return from this digression to the real object of our discussion, namely that the nutritive solutions of organisms must be very dilute and consist of the split products of the complicated compounds of which the organisms consist. The examples given sufficiently illustrate this statement. The nutritive medium of our body cells is the blood, and while we take up as food the complicated compounds of plants or animals, these substances undergo a digestion, _i. e._, a splitting up into small constituents before they can diffuse from the intestine into the blood. Thus the proteins are digested down to the amino acids and these diffuse into the blood as demonstrated by Folin and by Van Slyke. From here the cells take them up. The different proteins differ in regard to the different types of amino acids which they contain. While the bacteria and fungi and apparently the higher plants can build up all their different amino acids from ammonia, this power is no longer found in the mammals which can form only certain amino acids in their body and must receive the others through their food. As a consequence it is usually necessary to feed young animals on more than one protein in order to make them grow, since one protein, as a rule, does not contain all the amino acids needed for the manufacture of all the proteins required for the formation of the material of a growing animal.[15] [15] This fact was thoroughly established by Mendel and Osborne. A summary of their work is given in Underhill, F. P., _Physiology of the Amino Acids_, 1916. 3. The essential difference between living and non-living matter consists then in this: the living cell synthetizes its own complicated specific material from indifferent or non-specific simple compounds of the surrounding medium, while the crystal simply adds the molecules found in its supersaturated solution. This synthetic power of transforming small “building stones” into the complicated compounds specific for each organism is the “secret of life” or rather one of the secrets of life.