THE EMBRYOLOGY AND ETIOLOGY OF TUMOURS 77
Matters would appear to be complicated in some cases by the existence of two well-marked portions in a normal life-cycle: the asexual one, represented by the chorion (trophoblast); and the sexual one, taken up by an embryo, or Metazoon. Some tumours—some ovarial teratomata, described by Wilms—exhibit attempts to include the whole life-cycle, in that, along with a pathological bizarre embryo, cancer, representing the asexual generation, is encountered. In other tumours, again, there would appear to be nothing of the asexual generation; and still others, the malignant cancers, confine themselves entirely to being abnormal manifestations of the asexual portion—the first part of the life-cycle.
The comparative embryology of tumours may now be considered more in detail. The seed or seeds of tumours are unquestionably some or other of the vagrant (or if in ovary or testis, persistent) primary germ-cells, treated of at greater length in the writer’s works upon the germ-cells. So far (1911) they have only been found from fishes to reptiles ; but from various considerations it is not open to doubt that they occur even in the highest vertebrates, and in man himself. Apparently they have been noted by Roux and Barfurth in the frog, and by me in the salamander (S. maculosa). In all the embryos yet studied by me under a certain age—i.e., within the limits during which germ-cells are easily found in embryos— no single embryo examined has been devoid of them.
The mode of the development and the life-cycle, in practically all its details, are the same in mammals as in fishes, and unquestionably the whole organization and development of man follow closely along, but along higher lines than, those of a fish. It is therefore concluded that, could one but hit upon some easy method of distinguishing germ-cells during the early development of man and