classes of animals that a detached blastomere may reproduce the whole organism.
We have still to consider one very interesting kind of tumor, namely, the so-called chorionepithelioma malignum, which also owes its origin to irregularities in the activity of certain embryonic cells but which in some respects differs markedly from the other tumors considered so far. When the segmenting egg attaches itself to the uterine wall it does not only produce the embryo proper, but it gives also rise to certain cells which attach themselves to the uterine wall of the mother, and are mainly concerned in transferring food from the mother to the embryo, thus forming the embryonal placenta, the outer layer of which is called the chorionic. These chorionic cells, which are therefore of embryonic origin, migrate already under normal conditions deeply into the uterine wall; they may even penetrate into maternal blood vessels and be carried to other parts of the body of the mother. Usually these chorionic wander cells perish after some time in the mother, but occasionally they give rise to very malignant tumors which destroy the uterine wall, and form metastases (L. Fraenkel, Marchand).
Just as these chorionepitheliomata may be produced from the fertilized egg cell developing in the uterine wall, so they may occasionally owe their origin to egg cells which develop into embryomata in the germinal glands and especially in the testicle. Here likewise chorionepitheliomata may develop. In the structures which I found in the ovaries of guinea-pigs the greater part of the dividing egg cells formed in contact with and probably under the influence of the ovarian tissue of the mother placental tissue and especially migrating cells which penetrated occasionally even into the walls of the neighboring blood vessels.
We have now analyzed some of the factors concerned in the origin of tumors found in childhood and early adult life. We recognized that they are caused partly by abnormalities of embryonic development, partly by parthenogenetic development of germ cells. At least these are two of the factors concerned in their origin—whatever additional factors may be found in the future. However, the greater number of all tumors, especially the large majority of the typical cancers found in later life, owe their origin to different causes. We can appreciate these causes best, if we consider certain special kinds of cancer which are somehow associated with certain kinds of occupation.
In general the character of the occupation does not seem to have a marked influence on the incidence of cancer, although it seems that cancer in certain callings (agricultural and forest workers, textile and wood workers, domestic servants) is somewhat more frequent than in others (miners, soldiers, factory workers in general). There are however certain occupations in which a direct connection exists between the
