Fig. 4. (A) The dissolved oxygen (DO) concentration 100 µm inside the chorion in 31 d.p.f. rainbow trout embryos before hatching, and in the same embryos, the DO concentration in the boundary layer relative to the distance to the chorion. Values are means ± s.e.m., N=6. (B) The boundary layer gradient (% sat; to Ch, the difference between the DO in the free-stream and the DO at the surface of the chorion) compared with the gradient across the chorion (x Ch, the difference between the DO at the surface of the chorion and the DO inside the chorion).