Studies on Tumor Variability

Morphological and cytogenetical studies of H<sub>10</sub>Z tumor in various periods of growth in animals emphasize its heterogenous constitution formed by two groups of cell types, i.e. (1) ‘primary cell types’, the main constituents of tumor structure, well defined morphologically and genetically, which are gradually selected during tumor evolution; (2) ‘secondary cell types’, which are distinguished only on genetical analysis by the presence of certain new marker chromosomes. These cells are very scarce in the tumor and their distribution does not present significant variations during tumor growth. The conclusion is drawn that modification of tumor characteristics during its evolution in animals takes place by gradual selection of certain primary cell types. The possibility of tumor variant appearance in special medium conditions, by the occurrence of the induction-selection phenomenon in a secondary cell type, is discussed.