The hatching synchrony of embryos within individual egg masses was examined in order to understand the factors that influence the rate of embryonic development ofLymnaea.When reared at 25°C, individual embryos from single egg masses hatch at various times from 9–16 days after first cleavage. The peak time for eclosion is on the 11th day of embryonic development, when approximately 30% of the embryos hatch. Hatching time correlates with the position of the embryo within the egg mass; embryos near the center of the mass hatch later than those on the periphery. This spatio-temporal pattern of hatching does not reflect any pattern of initial differences in the sizes of the individual egg capsules or in the sequence of eggs undergoing first cleavage. Instead the pattern of hatching appears to correlate with the relative exposure of the embryos to the external environment. Furthermore, isolation of eggs from the mass results in shorter times to peak hatching and much greater synchrony in hatching: i.e. 90% of the embryos hatch over a 2-day period (from 9 to 10 days after first cleavage). Together these findings suggest that the pattern of hatching observed in intact egg capsules results from positional differences of the embryos for the exchange of substances with the environment.