THE EMBRYOLOGY AND ETIOLOGY OF TUMOURS 85
Through other species of Styhfer, Entocolax ludwigii is reached. This is parasitic in the body-cavity of a sea-cucumber (a Holothurian), and with the endoparasitism the shell, mantle, gills, and sense organs disappear. The series culminates in Entoconcha mirabilis, parasitic in the sexual organs of a sea-cucumber (Synapta digitata). This is nothing but a worm-like sac, containing (hermaphrodite) sexual organs. In its organization there is nothing whatever of molluscan characters, and its true nature can only be made out in one or other of two ways: by the comparative anatomy of the series of such parasitic mollusca, which reveals all the stages from highly organized forms down to absolute reduction of most or all of the organs, or by the study of the development; for, as Johannes Muller showed in one of his classic works, Entoconcha is a true gasteropod, with shell and other organs.
As it is impossible to follow the whole development of a tumour from its first start, and as many of them attain only a very low degree of embryonic differentiation, the series of forms in them, leading from the highest to the lowest, from the most organized to the simple ones, can only be followed after the method of comparative anatomy, and this is the plan adopted by Wilms in his researches. From the results of a study of tumours made in this way, and with the facts established by the writer’s researches upon the germ-cells, and the course of the cycle of development of the higher animals, the following conception* of the true nature of a neoplasm or tumour is obtained.
A tumour is a more or less reduced, more or less incom-
* It should be specially noted that in this definition malignant tumours (carcinoma and sarcoma) are excluded—only tumours representing the sexual generation are included.