Fig. 5. The embryo stir-bar hypothesis. (A) In the absence of rotation, a large oxygen concentration gradient will form due to the metabolic consumption of oxygen by the embryo (depicted as stationary stir bar) and the unstirred boundary layer of high oxygen concentration below the capsule surface. (B) Embryonic rotation and ciliary activity (depicted as rotating stir bar) function to mix the capsular fluid, causing a reduction in the size of the oxygen gradient inside the capsule, a higher concentration of oxygen at the embryo surface, and enhanced transfer of oxygen into the egg capsule. The arrow size in A and B represents the relative size of the oxygen gradient across the egg capsule membrane. Stir bar length is not representative of embryo size (see Fig. 1).