PRESIDENTIAL ADDRESS. 677 
the disturbance of normal gravitational relations. This finds expression in the 
accumulation of homogentisinie acid as the result of the formation of an anti- 
oxidative substance which arrests the complete disruption of tyrosin in the cells. 
Whether this is the immediate cause of the geotropic movement, or merely a con- 
comitant of it, we cannot settle at present. But it is of the highest interest to 
know that chemical change is initiated as a result of the external gravitational 
impulse, even when the latter is of too short duration to produce an actual 
geotropic movement. And although we may not at present be able to identify 
the exact material which is directly concerned in these stimulatory or formative 
processes, we have, as it seems to me, irresistible evidence in favour of its real 
existence, It is more than mere analogy that leads us to believe that the various 
kinds of galls, for example, that may be produced on an oak leaf owe their formation 
to the specific interference of the secretion of the grub with the higher metabolic 
processes going on in the cells of the leaf. 
I have alluded to the different conditions under which given reagents may 
interact, and these may in turn very materially affect the final result by modifying 
the course of ihe reaction itself, We are coming to realise the fact that the 
physical state of the cellular constituents exercise an important influence on the 
course of chemical activity manifested within their range. We all know what an 
important part water plays in ordinary chemical reactions, but the water question 
assumes a special prominence when the reactions are going on in a colloidal 
matrix, or rather in a mixture of colloids, such as the various proteins that occur 
in the cell. Questions of rates of diffusion, physical adsorption, and the like have 
to be taken into account; and beyond all these there remain the series of remark- 
able electrical relations which the proteins exhibii, as well as those changes in 
surface-tension that are, in part at least, connected with them. 
It is impossible to resist the belief that a closer study of the physico-chemical 
changes that accompany a nuclear division will yet throw much light on the 
mechanics of this wonderful process. Indeed we already possess some data which 
are serving as starting-points for further investigation, and they have placed some 
of the known facts in a very suggestive light. 
It has often been urged as a reproach against the histological methods 
employed in the study of the cell that all such investigations can, after all, only 
give information as to the character of coagulations or precipitations. . Of course 
this is perfectly true ; but provided we have sufficiently good grounds for enabling 
us to feel confident that the precipitation or coagulation faithfully maps out the 
positions originally occupied by the respective colloids during life, there is no real 
force in the objection. No one would call in question the accuracy of a photo- 
graphic negative on the ground that after development it no longer consisted of 
the actual substances which had been formed in the film by the exposure to the 
action of light. All that is required is that the deposited silver shall accurately 
express the limits of, and be proportionate in amount to, the alteration in the | 
composition of the salt which was produced when the plate was exposed in the 
camera. 
Much of the general detail of a nuclear division can be followed even 
in the living cell, and we therefore possess direct as well as indirect means of 
testing the degree of accuracy with which the fixed preparation represents the 
original pattern of distribution of the colloids within the cell. No one who has 
studied the behaviour of artificially prepared mixtures of colloidal proteins and 
nucleins after ‘fixing’ and staining them can entertain reasonable doubts as to 
the substantial identity of the structures visible in a well-fixed cytological 
preparation with those present during life. For the substances, even in these 
artificial mixtures, keep remarkably distinct, as indeed Fischer showed some 
years ago, 
Few things are more striking than the remarkable series of evolutions passed 
through by the linin, and by the chromosomes which finally emerge from it during 
the progress of a mitosis. We have clear evidence that the nucleus at this period 
is the seat of rapid chemical change. ‘he process of distribution of the nuclein 
within the linin is sufficient proof in itself of this, But we have also, I believe, 
