14 THE PRESIDENTIAL ADDRESS 
vitamin. For the display of an influence upon growth, however, 
the exact details of the vitamin’s proper structure must be established. 
Now turning to vitamin D, of which the activity is more specialised, 
controlling as it does the growth of bone in particular, we have 
learnt that the unit elements in its structure are again isoprene 
radicals ; but instead of forming a long chain as in vitamin A they 
are united into a system of condensed rings. Similar rings form 
the basal component of the molecules of sterols, substances which 
are normal constituents of nearly every living cell. It is one of 
these, inactive itself, which ultra-violet radiation converts into 
vitamin D. We know that as stated each of these vitamins stimu- 
lates growth in tissue cells. Next consider another case of growth 
stimulation, different because pathological in nature. As you are 
doubtless aware, it is well known that long contact with tar induces 
a cancerous growth of the skin. Very important researches have 
recently shown that particular constituents in the tar are alone con- 
cerned in producing this effect. It is being further demonstrated 
that the power to produce cancer is associated with a special type of 
molecular structure in these constituents. ‘This structure, like that 
of the sterols, is one of condensed rings, the essential difference 
being that (in chemical language) the sterol rings are hydrogenated, 
whereas those in the cancer-producing molecules are not. Hydro- 
genation indeed destroys the activity of the latter. Recall, however, 
the ovarian hormone estrin. Now the molecular structure of cestrin 
has the essential ring structure of a sterol, but one of the constituent 
rings is not hydrogenated. In a sense therefore the chemical nature 
of estrin links vitamin D with that of cancer-producing substances. 
Further, it is found that substances with pronounced cancer-pro- 
ducing powers may produce effects in the body like those of cestrin. 
It is difficult when faced with such relations not to wonder whether 
the metabolism of sterols, which when normal can produce a sub- 
stance stimulating physiological growth, may in very special circum- 
stances be so perverted as to produce within living cells a substance 
stimulating pathological growth. Such a suggestion must, however, 
with present knowledge, be very cautiously received. It is wholly 
without experimental proof. My chief purpose in this reference to 
this very interesting set of relations is to emphasise once more the 
significance of chemical structure in the field of biological events. 
Only the end results of the profound influence which minute 
amounts of substances with adjusted structure exert upon living cells 
or tissues can be observed in the intact bodies of man or animals. 
It is doubtless because of the elaborate and sensitive organisation of 
chemical events in every tissue cell that the effects are proportionally 
So great. 
It is an immediate task of biochemistry to explore the mechanism 
