Current concepts of the relation between body size and cell numbers are reviewed and analyzed. The occurrence of eutely in the animal kingdom from Protozoa to the prochordates is reviewed and the following conclusions are drawn: Constancy seems to be the outgrowth of a principle of development from the physiological rhythms which mark most life processes. Eutely is of 2 classes: (1) that in which the entire soma conforms to a fixed pattern of cell number and arrangement; (2) that in which only a limited number of organs or structures have fixed cell formulas. Inconstancy is likewise of 2 types: (1) primitive inconstancy, the consequence of indeterminate cleavage, and (2) derived inconstancy wherein eutely once established is destroyed by the addition of a stage of nuclear readjustment and fragmentation accompanying senescence.