The mechanism of hardening of the salmonid egg membrane after fertilization or spontaneous activation.

T H E embryonic development of fishes proceeds under the protection of rigid egg membranes which preserve the embryo from mechanical injury. Salmonid fishes bury their eggs in sandy and stony ground so that they are particularly liable to mechanical damage. This is apparently the reason why the membrane of the salmonid embryo is extremely strong, resisting a load of 3-4 kg. per ovum (Gray, 1932; Hayes, 1942,1949; Hayes & Armstrong, 1942; Zotin, 1953a). This mechanical property of the membrane does not appear immediately after fertilization or activation but is preceded by a whole set of processes which are elicited in the membrane by external factors and by the fertilized or activated egg itself. Thus, according to Manery, Fisher, & Moore (1947), hardening of the egg membranes in the speckled trout sets in 2 hours after the release of the egg into water. Ca ions have been shown to be of great significance for membrane-hardening in Salvelinus fontinalis (Manery, Fisher, & Moore, 1947; Warren, Fisher, & Manery, 1947; cf. also Hoar, 1957) and Oncorhynchus keta (Kusa, 1949 a, b). In distilled water, membrane-hardening is slower than in ordinary water and is completely blocked when Ca ions are bound by 0 01 N sodium citrate or 002 N sodium oxalate. Fertilized eggs of the lake salmon and trout have been shown by Zotin (1953a, 1958) to secrete substances which are indispensable for the increase of membrane toughness. The secretion of these substances requires a short exposure to water to activate the egg. The activating effect of water upon unfertilized salmon eggs has often been described (K. Yamamoto, 1951; Soin, 1953; Disler, 1954). The secretion of substances eliciting hardening or solidification of egg membranes after fertilization or activation has been described in some sea-urchin species (Motomura, 1941; Runnstrom, 1947, 1952) and in sturgeons (Zotin, 1953W as well as in the teleostean fish Oryzias latipes (Nakano, 1956). Solidifi-