Incubated eggs of berried female crab Portunus pelagicus are hanged to the pleopodal setae by an attaching membrane. Each egg is protected by inner chorion and middle chitinous membrane. The middle chitinous and the outer attaching membranes form together a distinct case surrounding each egg. This case resists acids and alkalies and appears impermeable to water, salts and dye substances. The histochemical results provide evidence as to the origin of the chorion and suggest its function. The chitinous membrane is tanned, calcified and contains chitin. The attaching membrane is of lipoid nature and appears to be rich in polyphenols and phenolase enzyme suggesting the presence of phenolic tanning. The histochemical results confirm the cuticular origin and nature of the egg case. The tegumental glands of the pleopods are involved in the formation of the outer attaching membrane and/or at least the phenolase enzyme necessary for phenolic tanning. Larval release is a result of molting if the egg case is considered as a part of the outer cuticular exoskeleton of Portunus.