only for the chance, and that there is besides something which keeps it in, which holds it back, which stops it? We call this stopping physiological function—inhibition; we say that the growth of the skin is inhibited; though in the deep part of the skin all the time there are the cells ready to grow as soon as that power of inhibition is taken away; while it is active they will not grow. The simple blister tells us all that. There is, then, a power of regulation which expresses itself in this inhibitory effect. When a salamander has its tail cut off by the experimenter and the new tail grows, just enough is produced. The new tail is like the old. The tissues grow out until the volume of that which is lost is replaced, and then they stop. But if the tail should be cut off again, regeneration would occur again. The experiments may be repeated many times over. It indicates to us that always the growing power is there, but it is held in check. What that check may be is one of the great discoveries we are now longing for. The discover}', when made, is likely to prove of great practical importance. The phenomenon of things escaping from inhibitory control and overgrowing, is familiar. Such escapes we encounter in tumors, cancers, sarcoma and various other abnormal forms of growth that occur in the body. They are due to the inherent growth power of cells kept more or less in the young type, which for some reason have got beyond the control of the inhibitory force, the regulatory power which ordinarily keeps them in. No picture of the growth or development of the living animal would be complete if it confined its attention only to the power of growth in relation to cytomorphosis. It must also include the contemplation and study of this regulatory power of the organs. Experiments are being made in many places, minds are at work in many laboratories upon this problem of regulation of structure and growth. Much is to be hoped from such researches; not merely insight into the normal development, but insight also into the abnormal. Nothing, perhaps, is more to be desired at the present time than that we should gain scientific insight into the regulatory power which presides over growth. It would be of immense medical importance. Could we understand it, and could we from our understanding derive some practical application of our scientific discoveries in this field, we could say of it justly that it was as noteworthy a contribution to medical knowledge as the discovery of the germs of disease, and would doubtless prove equally beneficial to mankind. Although, then, the study which I have been laying before you must necessarily seem in many respects abstruse and far away from practical applications, we learn that it is not really so, and that it leads by no very remote path to the consideration of problems the useful applications of which are immediately obvious to every one.
We find in the process of regeneration that it is always the young cell which plays the principal part. This is beautifully illustrated in
