A number of authors have investigated the oxygen consumption and carbon dioxide production of the amphibian embryo at various stages of its development, with results of considerable interest. But such investigations throw no light upon the properties of the individual regions of the embryo at various stages, although the work of experimental embryologists has furnished us during the last 20 years with fundamental information about these regions and the part they play in the morphogenesis of the organism. In particular, the phase of gastrulation, during which are formed the germ-layers of classical embryology, and which involves the action of the primary organiser in determining the main axial structures of the embryo, merits the closest study.Only recently have sufficiently delicate chemical methods become available for attacking this problem. Since Rehberg1 developed the first ultra-micro burette, these methods have been greatly extended by the work of Linderstr⊘m-Lang, Holter, and their collaborators at...