When connective tissue cells have been cultivated for a certain length of time in a medium which has been repeatedly changed, a definite relation arises between the rate of growth of the cells and the composition of the medium. It is possible, by adding to the culture medium a given quantity of certain substances, such as embryonic juices, to foresee the extent to which a fragment of tissue composed of a given strain of cells will increase in a given time. The rate of growth of a strain of cells can be accelerated or retarded by the addition to the medium of activating or retarding substances. The dynamic condition of a strain of connective tissue cells, which have been living in a given medium for some time, is not a definitely acquired characteristic, but a temporary state, and is the product or function of the medium in which the cells are living, and is readily modified merely by altering the composition of the medium. A knowledge of the characteristics of the growth of connective tissue described has led to a new result,-the indefinite proliferation of a strain of connective tissue cells outside of the organism. The strain of connective tissue originally obtained from a fragment of chick embryo heart, which had been pulsating in vitro for 104 days, was still actively alive after sixteen months of independent life and more than 190 passages. The rate of proliferation of the connective tissue sixteen months old equalled and even exceeded that of fresh connective tissue taken from an eight day old embryo. It appears, therefore, that time has no effect on the tissues isolated from the organism and preserved by means of the technique described above. During the sixteenth month of life in vitro the cells increased rapidly in number and were able in a short time to produce a large quantity of new tissue. This fact, therefore, definitely demonstrates that the tissues were not in a state of survival, as was the case in certain earlier experiments, but in a condition of real life, since the cells of which they were composed, like microorganisms, multiplied indefinitely in the culture medium.