Salt and Water Regulation in the Embryos of Freshwater Pulmonate Molluscs

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Abstract
1. The envelope and capsular membranes which separate the eggs from the external water are freely permeable to water and inorganic ions. 2. The capsular membrane of B. sudanica is permeable to sucrose (M.W. 342) but not to raffinose (M.W. 504), that of L. stagnalis is permeable to raffinose but not to Carbowax (polyethylene glycol, M.W. 3000-3300). 3. By means of 22Na it was shown that, as expected, the concentration of sodium in the capsular fluid of B. sudanica is approximately equal to that in the external water. 4. The initial colloid osmotic pressure of the capsular fluid of B. sudanica is equivalent to that of approximately 1.5 mM/l. solution of non-electrolyte and of L. stagnalis to that of 4-5 mM/l. 5. Observations on embryos of B. sudanica extracted from the capsular fluid and kept in lake water support the contention that, at least from late cleavage stages onwards, they are capable of active salt and water regulation.

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