From cleavage to hatching Lymnaea stagnalis (L.) embryos exhibit net uptakes of radioactive sodium, potassium and calcium greater than the amounts of these ions initially present in the capsular fluid, implying the early development of mechanisms for ion uptake from the dilute external water. The rates of net uptake increase very rapidly during trochophore to veliger stages when the main volume increase and shell formation takes place.

22Na influx increases dramatically at first cleavage from 0.03 to 0.04 pmol min−1 in the hour before cleavage to 0.23 pmol min−1. A further increase occurs in early cleavage and for decapsulated morula to gastrula stages the influx is estimated to be 2.75 pmol min−1. 22Na efflux rates are also high in decapsulated morulae, blastulae and gastrulae (minimal estimates 1.40–3.35 pmol min−1) indicating a rapid exchange of sodium between the embryos and the medium. In sodium-free medium the 22Na efflux from decapsulated blastulae is 0.5 pmol min−1, suggesting that exchange diffusion is a large component of the fluxes. The characteristics of 22Na efflux indicate that sodium does not exchange with the medium as a single compartment.

Expansion of the ‘recurrent’ cleavage cavity of early stages is accompanied by a rise in exchangeable sodium content from 5 pmol in embryos without cavities to 11–19 pmol in embryos with normally expanded cavities. Experimental expansions of the blastocoel of morula-blastula stages produced by cyanide are also accompanied by great increases in embryo exchangeable sodium content.

A general model is proposed for the movements of sodium and water in the embryos and this provides a mechanism for hyperosmotic regulation by the embryos.

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