Connective tissue cells of chick embryos and cells of a chicken sarcoma, proliferating in vitro, soon render acid the plasma about them, but they nevertheless continue to grow well. Evidently the tissue cell will withstand a considerably greater change in the reaction of the fluids about it than has usually been supposed. Under conditions of in vitro life in plasma, which do not provide for an artificial circulation, the acid produced by growing tissues diffuses only slowly and is subject to little dilution from this source. About tissues which grow very rapidly in vitro, as, for example, tumor tissues, there must be a marked concentration of metabolic products, and this may largely account for the poor results of attempts at the continuous propagation of such tissues in vitro.