The conclusion to which the foregoing experimental results point is that thyroid ingredients, no matter from what class of vertebrates the gland be taken, produce essentially the same result when given to ciliate protozoa (Paramæcium and Stylonichia) as a food or as a factor in the medium in which they live, viz., increased division rate. The tissue which has hitherto been used in experimental work along this line has, we believe, always been taken from mammals. We think it safe to say that, no matter how far apart taxonomically, or how distantly related phylogenetically the "higher" and "lower" members of the vertebrate phylum may be, certain physiological qualities in the thyroid glands are constant and similar in all. Sufficient difference exists between the potential of the thyroid secretion of one vertebrate class and that of another, so that, if studies of the normal value of this gland are being made, glands from the same class, if not from the same genus and species of animal as the one under observation, may well be employed.