THE EMBRYOLOGY AND ETIOLOGY OF TUMOURS 75
with the remaining complicated” three-layered” tumours, which, as due to developmental abnormalities of a single embryo, are stated to be monogerminal. As, according to Wilms, all possible transitions between the most complicated embryomata and the simpler tumours exist, there would appear to be no grounds for this and similar increases in the hypotheses.
Apart from its entirely hypothetical character, its lack of support in facts of embryology, and its continual and unnecessary multiplication of causes, the theory of germ-shuntings labours under other difficulties. It is not easy to conceive any adequate cause for such shuntings during development, and the difficulty is greatly increased when cases of multiple tumours in very different parts of the body, each of which requiring one or more shuntings at some period or other, are taken account of; for with them a normal development would appear to be quite out of question. But, granted the possibility of such shuntings, the real difficulties begin. What causes such a shunted germ, ignoring all laws of differentiation, to embark upon a career of damage, riot, and destruction of its own? To take an instance from Wilms, typical of many such: the germ of an osteo-sarcoma will be a cell or germ of the periosteum of some bone. Normally, like its fellows, it ought to have contributed to the formation of that bone. Instead, thereof, at some period or other, after lying dormant, it breaks all bounds, and proceeds on a line of development of its own. This is such that, unless brought to a stop by some extrinsic cause or other (operation or death of the host), it may be the parent-cell of more progeny than all the other bone-producing cells in the body! In fine, Wilms ascribes to his shunted germs far greater embryological potentialities than Nature ever endowed them with. On the other