Salt and Water Regulation in the Embryos of Freshwater Pulmonate Molluscs

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Abstract
1. By means of 22Na it is shown that the embryo takes up sodium from the water via the capsular fluid so that its sodium content is increased more than seven times between the blastula and hatching. 2. Experiments with water of low sodium content (Text-fig. 3): (a) suggest that sodium is taken up by the preblastula embryo, and (b) demonstrate that the embryo can develop with a sodium content less than one-seventh of the normal. 3. Embryos at blastula or even cleavage stages, when deprived of access to external water and ions by mounting over paraffin, can still accomplish the greater part of their development, thus confirming the conclusion 2(b). These experiments also suggest other ions than sodium can be considerably reduced without harm to the embryo. 4. There is evidence, however, that the ecologically inhibiting effect of very low salinity water is due to the low level of ions other than sodium. 5. Reduction of dissolved oxygen to 0.1-0.2 mg./l. for 12 hr. completely inhibits sodium uptake and visible development. These are both reversible but, though the normal rate of development is resumed at once, the rate of sodium uptake is not restored for several days after re-aeration. The ultimate effect of this treatment on some, but not all, embryos is an upset of water balance shown by swelling and the suppression of the final stages of development.

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