THE EMBRYOLOGY AND ETIOLOGY OF TUMOURS 91
But, surely, now, the etiology of cancer is as clear as that of the tumours in general!
Very shortly after the date of writing the above, a little further light was obtained in another direction. It is a natural question to ask, “Can any and every primary germ-cell undergo abnormal development, or is this power limited to certain of them ? “ A full reply to this would entail prolonged investigation into the developmental problems of identical twins, triplets, etc. For a long time now (1911) this matter has engaged the writer’s attention more or less, but though some landmarks can be recognized, the end is not yet in sight. So far as I can see, the whole doctrine of the tumours and cancer centres in the problems of identical twins, triplets, etc.—in fine, in the question of the number of embryos which may arise from one egg, and therefore be contained in one chorion or trophoblast.
A full discussion of identical twins, etc., must be reserved for another occasion, in a projected book upon heredity. Here it need only be stated that their occurrence is probably more frequent than has been supposed hitherto. By competent authorities it has been estimated that in man identical twins form 25 per cent. of all twins. Their comparative frequency alone is against the idea of their occurrence being due to, say, a chance division of the developing egg. The absurd supposition of their etiology by “ the splitting of a germ” was exposed by me in a letter in the Lancet, January 7, 1905, p. 56. Extremely improbable, if not impossible, is the origin of one of them from a fertilized polar body. As little can one hold this as accept the idea of “chance” in the development. In some other mammals identical twins would appear to be very common. Thus, in the sheep, where the total number of young is usually two or three,