NUTRITION 



261 



Euglena, is unable to grow and multiply in a pure saline medium, although all the 

 elements supposed to be necessary are present. If, however, a trace of tyrosine be 

 added, growth is vigorous. It seems that the tyrosine does not serve as a food- 

 stuff, since the active amount is so small (0'4 mgm. in 10 c.c.). Moreover, no 

 growth occurs in the dark, so that the organism obtains its food from the carbon 

 dioxide of the air and the mineral substances of the culture fluid. The tyrosine 

 disappears, it is true, but this is due to the action of bacteria, unavoidably present 

 in small numbers. See also E. J. Allen (1914) on the growth of diatoms. 



The manganese and zinc required for the normal growth of Aspergillus 

 may also be mentioned in the present connection. 



As to the mode of action of these "hormones" or " catalysts " we are as yet in the dark ; 

 as indeed as to that of the 

 similar substances in the 

 "internal secretions." Some 

 facts which bear on the ques- 

 tion will be found in Chapter 

 XXIV. and, as to the nature 

 of catalysis, in Chapter IX. 



There is one possi- 

 bility that has not been 

 referred to as yet. We 

 saw in Chapter V. how 

 important a part the cell 

 membrane plays in vital 

 phenomena. Now, the 

 actual amount of material 

 contained in these mem- 

 branes is almost infini- 

 tesimal, yet it might well 

 be that there is something 

 absolutely necessary to 

 its proper constitution, a 

 substance which may be 

 gradually lost in the pro- 

 cess of activity. One 

 thinks of Clark's experi- 

 ments (1913, 2) on the 

 effect of prolonged per- 

 f usion with a simple saline 

 solution, which washes 

 away something which 

 can be replaced by an 

 ether extract of the dry 

 residue of an alcoholic 

 extract of dry serum (see 

 page 211 above). The 

 properties of lecithin may occur to the reader, but this, as well as cholesterol, seems 

 to be excluded by the experiments of Stepp, who found neither of these, nor indeed 

 other known " lipoids," capable of replacing the active substance in milk. 



An rmportant conclusion with respect to the general theory of nutrition is 

 to be drawn from the various facts given in the present section. As was 

 pointed out by Starling and myself (1906, p. 696) and by Hopkins (1906, p. 395), 

 independently, it is not sufficient to estimate the value of a diet, as to whether 

 it is an adequate one or not, merely by its calorie value or its content in fat, 

 carbohydrate, and protein ; the presence or absence of the small quantity of 

 the " accessory factors " or " hormones " must also be taken into account. 



THE METABOLISM OF PROTEINS 



Our next task is to discuss briefly the changes undergone by the three classes 



FIG. 73. PORTRAIT OF EMIL FISCHER. 



