[Origin of embryonal features of the neoplasm and tumor progression]

It is immature elements of both proliferating and nonproliferating tissues that serve target cells in oncogenesis. Fould's concept of the independant development of tumor characters is supplemented with the idea that one must discriminate between fundamental festures, common for both benign and malignant neoplasms (uncontrolled growth), and secondary ones manifesting malignancy. Some of the latter are thought to be obligatory (invasiveness with destructive growth and the systemic effect) and others--facultative (capacity of metestasizing, cataplasia, etc).