Implanted mixtures of mouse embryo and tumor sometimes result in an excellent growth of both. To assure this, it is necessary that the tumor selected be one which proliferates slowly, because more active ones prevent the development of the fragments of embryo. Transplanted embryonic tissue (of the type used) does not, at its best, grow as rapidly or in so large a percentage of hosts as some tumors.
The morphological relations between tumor and embryonic tissue in the mixed graft are often intimate. Apparently either may adapt the other to its structural purposes. Occasionally a direct union takes place between cancerous epithelium and that of the embryo, with result in pictures suggesting an origin of one from the other. This observation has considerable significance in view of the current reliance upon just such histological data to prove that cancer arises from normal epithelium.