664 TRANSACTIONS OF SECTION I. 
agents, which are always present, or potentially present. If the available 
experimental evidence be honestly appraised, it points to the conclusion that 
only to hydrolytic processes are the complexes unstable. Under the conditions 
of the body they are, while intact, resistant to other types of change, their 
hydrolytic products being much more susceptible. Since hydroclastic agents 
are present in the cell we must suppose that there is, at any moment, 
equilibrium between the complexes and their water-soluble hydrolytic pro- 
ducts, though the amount of the latter present at any moment may be very 
small. Now, I think we are entitled to look upon assimilation and dissimiia- 
tion, when very strictly defined, as being dependent upon changes in this 
equilibrium alone. They are processes of condensation and hydrolysis 
respectively. Substances which are foreign’ to the normal constitution of 
the complexes—and these comprise not only strictly extraneous substances, 
but material for assimilation not yet ready for direct condensation, or 
metabolites which are no longer simple hydrolytic products—do not enter or 
re-enter the complexes. They suffer change within the cell, but not as part of 
the complexes. When, for instance, a supply of amino-acids transferred from 
the gut reaches the tissue cell, they may be in excess of the contemporary limits 
of assimilation; or, once more, individual acids may not be present in the 
harmonious proportion required to form the specific proteins in the cell. Are 
we to suppose that all nevertheless become an integral part of the complexes 
before the harmony is by some mysterious means adjusted? I think rather that 
the normality of the cell proteins is maintained by processes which precede 
actual condensation or assimilation. Conversely, when the cell balance sets 
towards dissimilation, the amino-acids liberated by hydrolysis suffer further 
change outside the complexes. So when a foreign substance, say benzoic acid, 
enters the cell, we have no evidence, experimental or other, to suggest that. such 
a body ever becomes an integral part of the complexes. Rather does it suffer 
its conjugation with glycine in the fluids of the cell. So also with cases of 
specific chemical manufacture in organs. When, for instance, adrenaline—a 
simple, definite crystalline body—appears in the cells of the gland which 
prepares it, are we to suppose that its molecule emerges in some way ready- 
made from the protein complexes of the gland, rather than that a precursor 
derived from a normal hydrolytic product of these proteins or from the food 
supply is converted into adrenaline by reactions of a comprehensible kind, 
occurring in aqueous solution, and involving simple molecules throughout? 
While referring to adrenaline, I may comment upon the fact that the extra- 
ordinarily wide influence now attributed to that substance is a striking 
illustration of the importance of simple molecules in the dynamics of the body. 
It should be, of course, understood, though the consideration does not affect 
the essential significance of the views I am advancing, that the isolation of 
reactions in particular phases of the cell is only relative. I have before empha- 
sised the point that the equilibrium of the whole system must, to a greater or 
less degree, be affected by a change in any one phase. A happening of any kind 
in the fluid phases must affect the chemical equilibrium and, no less, the physico- 
chemical equilibrium, between them and the complexes or less fluid phases. A 
drug may have an ‘action’ on a cell, even though it remain in solution, and it 
may have a specific action because its molecular constitution leads it to intrude 
into, and modify the course of, some one, rather than any other, of the 
numerous simple chemical reactions proceeding in the cells of different tissues. 
But I must now turn from consideration of the reactions themselves to 
that of their direction and control. It is clear that a special feature of the 
living cell is the organisation of chemical events within it. So long as we are 
content to conceive of all happenings as occurring within a biogen or living mole- 
cule all directive power can be attributed in some vague sense to its quite 
special properties. 
But the last fifteen years have seen grow up a doctrine of a quite different 
sort which, while it has difficulties of its own, has the supreme merit of 
possessing an experimental basis and of encouraging by its very nature further 
experimental work. I mean the conception that each chemical reaction within 
the cell is directed and controlled by a specific catalyst. I have already more 
than once implicitly assumed the existence of intracellular.enzymes. I must now 
consider them more fully. 
