The loosening of the egg capsule plug prior to escape of the veligers is shown to be chemically mediated in Nassarius obsoletus and N. trivittatus. 2. The hatching substance is not produced continuously during development, but rather in a short pulse beginning just prior to hatching and ending within 4 hours of escaping from the capsule, for N. obsoletus. 3. The hatching substance produced by the embryos of one species is effective only on the capsule plugs of that species, for the two species studied. 4. The substance is functionally short-lived, at room temperature in sea water, losing its potency within three hours after its secretion by N. obsoletus. 5. The observed rate at which N. obsoletus veligers leave their egg capsules is shown to be in close agreement with the rate predicted from an equation assuming random movements of individuals within the capsules.
