IN discussing the more recent experimental work upon the endocrine glands in their relation to amphibian metamorphosis, we cannot leave wholly out of account the earlier work that forms a basis for it; but the great volume of investigation in this field necessitates a limitation in an article of this character, more justified because of the fact that the writer published a survey article of this type in 1929 (Allen, 1929).For this reason the details of our study will concern chiefly such papers as appeared since that date, yet we must develop the basic principles of these earlier studies extending back to the year 1912 in order to lay our foundations.The pioneer workof Gudernatsch (1912), in which he demonstrated the fact that thyroid gland material fed to amphibians accelerates metamorphosis, was followed by studies by Allen (1916) and Hoskins, E. R. & M. M. (rgq), in which the con-