An understanding of the biochemical aspects of morphogenesis depends in part on careful descriptions of the metabolism of embryonic tissues. A preliminary stage in such studies is the elucidation of both the metabolic changes occurring during embryonic development and the metabolic differences existing between various embryonic regions. Accordingly the present work, which deals with one phase of the metabolism of the early chick embryo, is in p a r t an investigation of the changes in the respiratory rate of the area pellucida during development from the stage of the unincubated blastoderm to the 37-hour embryo (11-12 pairs of somites). The second and complementary part of the work is the comparison of the rates of oxygen consuniption in different regions of the head-process embryo. MATERIAL AND METHODS After a preliminary period of incubation at 38" C. ( * in a n electric, forced-draft incubator) blastoderms were removed * This work was carried out during the author's tenure of the Theresa Sessell Fellowship in Biology of Yale University for 1940-1941. The author is very grateful for the hospitality provided him by the members of the Osborn Zoologicxl Laboratory and especially f o r the generous advice and aid of Dr.