"s owe their existence to a population of persistently proliferating s which probably originate from the normal generative cells of that ... Normal stem cells ordinarily tend to give rise to non-dividing terminally differentiated cellular progeny; malignant stem cells, on the other hand, suffer an impairment of differentiation. Early in embryonic life, stem cells are developmentally versatile; the earliest ones, e.g., s in the mouse, are , or individually capable of forming an entire organism. s are exceptional tumors in that they contain a multiplicity of tissues, a characteristic implying that their stem cells arise from cells more developmentally primitive than is the case in other malignancies."