NUTRITION 257 



Whereas if the hydrolysis were allowed to proceed for seventeen hours, the 

 resulting product was useless. The only detectable difference was that the 

 tryptophane reaction was still present after six hours, absent after seventeen hours. 

 Abderhalden and Frank completely hydrolysed horse flesh by boiling with 

 sulphuric acid and then added 0-5 per cent, of tryptophane. This mixture was 

 adequate for dogs. It is impossible to say definitely what the function of 

 tryptophane is, perhaps for the elaboration of some internal secretion, as suggested 

 by Hopkins. 



The function of zinc in the growth of Aspergillus should also be referred to 

 under the head of accessory food-stuffs, since it appears from the work of Bertrand 

 and Javillier (1912, 2) that it enters as a constituent into the plant itself. Of 

 course, it is not necessarily implied that it forms a part of the chemical structure 

 of protoplasm itself. 



2. As Hormones. We have seen that gliadin, in the experiments of Osborne 

 and Mendel, although lacking in lysine and insufficient for growth, appears to be 

 adequate for maintenance, although the results of Hopkins and Neville (1913) 

 throw some doubt on its adequacy for any length of time even for this purpose. 

 Now there are other substances, of which a very minute amount only is required, 

 but in whose absence a diet, otherwise perfectly adequate even for growth, is 

 unable to preserve life. There are several sets of facts derived from different 

 kinds of phenomena which prove this statement. 



As many of these experiments were made on rats and mice, and some investigators hold 

 that these small animals are inappropriate for metabolism experiments, it is well to refer to 

 the circumstance pointed out by Hopkins (1912, p. 427) that for the kind of experiments in 

 question such small animals are especially valuable ; a number can be dealt with at the same 

 time, and the fact that their metabolic processes are rapid is important, especially for 

 experiments on growth. 



To turn to the experiments themselves, Hopkins (1912), one of whose curves 

 is given in Fig. 72 (page 254), showed that a trace of fresh milk added to a diet, 

 on which otherwise rats ceased to grow and ultimately died in about twenty-seven 

 days, made it perfectly normal. We notice that even maintenance at the weight 

 attained, except for a short time, is impossible without the addition. The active 

 substance in milk must have been present in extraordinarily minute amount. 

 Protein-free alcoholic extracts of milk solids, as well as the ether extract of thfe 

 dry residue of the alcoholic extract, containing no inorganic constituents, as 

 also the boiled watery extract of mangolds, were effective. The constituent is 

 therefore neither protein nor salts ; lactose can also be excluded, since the 

 addition of it to the diet is useless, and we have seen from Lunin's experiments 

 that a diet containing casein, butter, lactose, and salts is insufficient as a diet. 



We may make a rough calculation as to the maximum possible amount of the active 

 constituent in 2 c.c. of milk thus: milk contains only about 4 per cent, of ether-soluble 

 constituents, so that 2 c.c. would contain 0'08 g. Moreover Stepp's experiments (1909 and 

 1911) enable us to exclude the greater part of these. This observer fed mice with bread and 

 milk which had been extracted with alcohol and ether ; he found that Buch a diet kept them 

 alive for only three to four weeks, whereas if a minute amount of the dry residue from the 

 extracts were added, they lived indefinitely and appeared to be in normal condition. In 

 further work (1913) Stepp finds that ether alone does not extract the important constituent 

 from dry food, although alcohol alone does so. He also found that the addition of the neutral 

 fat of milk was ineffective ; as were also lecithin, cholesterol, cephalin, cerebron or phytin. 

 We may therefore subtract from the above 0'08 g. in Hopkins' diet, that part consisting of 

 fat, lecithin, cholesterol, etc. This would leave scarcely more than a few milligrams, if so 

 much. Other evidence is that boiled watery extracts of mangolds, which would not contain 

 more than a trace of lipoids, are active, as Hopkins found. We must therefore agree with 

 Hopkins that a catalytic or stimulating influence of some kind is more likely than that an 

 actual tissue constituent is supplied. 



Stepp apparently holds that a " lipoid " of some kind is responsible for the 

 result produced by the extracts in question. There are, however, other substances 

 present in bread and milk which are soluble in alcohol and even to some extent 

 in ether. 



Osborne, Mendel, Ferry, and Wakeman (1913) find that the "accessory 

 factor" for growth, contained in milk, goes into the fatty part of the butter. 

 This contains neither nitrogen nor phosphorus, so that the active substance is 



