By means of biomicrocinema films, the process of cell fusion was demonstrated in tissue cultures of the uterine endometrium, cancers of the stomach, urinary bladder and ascitic fluid from ovarian cancer. Tissue cultures of mammalian cells from human and animal sources show different life cycles at various stages of development. Tumor cells are believed to be hereditarily changed normal cells, which fuse with activated mesenchymal cells and bring about new cell populations with alien genetic forms. Environmental conditions produce changes in the life cycle of cells which may eventuate in an alternation of asexually and sexually produced generations. A basic cause of tumorous growth is believed to be due to changes in the normal life cycle of cells which have evaded control by the organism. With the transformation of cells come alteration in metabolism, morphology, antigenic structure, etc.