Abstract 1. 1. An optical-pressure vessel, permitting direct microscopic observation of living material while under hyperatmospheric gas concentrations, is described. This vessel was employed in a study of the effects of high oxygen concentrations on the early cleavage stages of the egg of the sand dollar, Echinarachnius parma. 2. 2. While oxygen pressures of 45–60 psi (abs.) caused an acceleration in the cleavage rate, pressures of 110–115 psi prevented further cell division after successful completion of first cleavage under oxygen pressure. This block to further cleavage can be reversed by exposing the arrested eggs to atmospheric air within four hours. Normal cleavage obtained with oxygen equivalents of an atmosphere of normal air and superimposed nitrogen presures to equal 110–115 psi, indicate that a direct effect by oxygen is responsible for the block, not a mechanically induced pressure effect. 3. 3. Exposing fertilized eggs to 110–115 psi of pure oxygen for periods up to 60 minutes, resulted in increasing the time before first cleavage could occur. Subsequent cleavages were not effected by such short exposure times and development proceeded normally. Exposure of fertilized eggs to 110–115 psi of oxygen for 120 minutes resulted in delays of second and third cleavage. The time required to complete a single cleavage was also prolonged by exposure to 110–115 psi of oxygen. 4. 4. Eggs that had undergone first cleavage in normal air and were then placed under 110–115 psi of oxygen showed a 400–500 per cent increase in the time required for second cleavage to occur. Such eggs invariably show developmental anomalies and eventually cytolyze upon return to normal air. 5. 5. Visualization of pressurized, cleaving eggs with infrared photomicrography revealed a distinct image of the achromatic figure at the 2 cell stage. It is concluded that, at the pressures employed, oxygen had no visible effect on the structural integrity of the blocked mitotic apparatus. The interference with cleavage is most likely mediated through an oxygen susceptible biochemical system associated with metabolic rather than structural mechanisms. The oxygen-induced effects on early cleavage are discussed with reference to the possible role of sulfhydryl groups.