Changes in Osmotic Pressure and Swelling in Horseshoe Crab Embryos During Development: (horseshoe crab embryo/swelling/water influx/perivitelline fluid/osmotic pressure).

Water influx accompanying the swelling of embryos during normal development of horseshoe crabs, Limulus polyphemus and Tachypleus tridentatus, following a rupture of the chorion, was analyzed. The increase in volume of perivitelline fluid during deveopment was about 90 percent of the increase in total embryo weight. Considerable water discharge was observed on drying the embryos in air and a reversible water influx occurred with a second immersion in sea water, even though the embryos died as a result of this treatment. Since the osmotic pressure of the perivitelline fluids decreased markedly during development until the end of swelling, a close correlation between swelling and osmotic pressure was recognized. These results indicate that certain osmoactive substances may be produced in the perivitelline fluid at the initial stage of swelling.