The perivitelline fluid is an extracellular fluid found in the eggs of most gastropods and constitutes the main source of nutrition and defense for their embryos. It replaces the egg yolk of other animals, which in snail eggs is reduced to non-nutritive proteinaceous granules with putative enzymatic function. During embryonic development the perivitelline fluid is ingested macropinocytotically by the embryos and the resulting phagosomes fuse with β-granules containing hydrolytic enzymes, which digest the perivitelline fluid components.