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The mechanism of action of the hatching substance released by Ilyanassa obsoleta embryos was examined by studying the sequential ultrastructural and biochemical changes that occur in the egg capsule plug as it is dissolved during hatching. When release of the hatching substance is triggered by incubating prehatching embryos in KCl, the inner two layers of the capsule wall (L3 and L4) that extend into the apex to form the plug separate from one another, but only in this region. The hatching substance then dissolves material at the periphery of the plug so that an intact plug can be recovered. However, if the plug is left in contact with the hatching substance, both the thin, electron dense material of the inner layer (L4) and the 10 nm filaments of the adjacent layer (L3) that compose most of the plug are dissolved. The first step in the hatching sequence is mimicked by papain so that L3 and L4 can be separated for analysis on SDS-polyacrylamide gels. L3 contains four major proteins with molecular weights of 24,000-52,000 daltons while L4 contains a predominant 25,000 dalton protein. When isolated plugs are dissolved in crude preparations of the hatching substance and analyzed by polyacrylamide gel electrophoresis, there is evidence of only slight disappearance of one minor plug protein. Based on these findings, Ilyanassa embryos probably release several activities necessary to dissolve the plug, yet degradation of the plug occurs without hydrolysis of the major plug proteins.

HATCHING OF ILYANASSA OBSOLETA EMBRYOS: DEGRADATION OF THE EGG CAPSULE PLUG IN THE ABSENCE OF DETECTABLE PROTEOLYSIS OF THE MAJOR PLUG PROTEINS