RATES OF IONIC-DIFFUSION ACROSS THE EGG CHORION OF ATLANTIC SALMON (SALMO-SALAR)

The diffusion coefficients for H+, Na+, and Ca2+ across the Atlantic s almon chorion were determined experimentally to be 3.2 X 10(-5), 1.3 X 10(-7) to 2.2 X 10(-6), and 3.3 X 10(-7) to 3.8 X 10(-6) cm2 . s-1, r espectively, depending on the model used. The ratios of diffusion coef ficients for the three ions do not differ greatly from the relative mo bilities in free solution (ca. 73:1:1.2), although the diffusion rates of Na+ and Ca2+ through the chorion may be retarded relative to that of H+. The chorion's capacity for ion exchange is low, relative to mos t commercial ion exchange materials. The diffusion coefficients were u sed to estimate flux of H+ into salmon eggs under pH gradients expecte d in nature as reported in the literature. The cation exchange capacit ies of the chorion and perivitelline colloids are insufficient to buff er H+ entry for more than a few hours. Since pH of perivitelline fluid (pvf) remains higher than ambient pH, it is suggested that the embryo nic tissues must generate basic materials to neutralize acidity enteri ng the pvf.