The viviparous prosobranchLittorina saxatilishas a reduced capsule gland and lacks a jelly gland and, thus, eggs and embryos of this species are covered only by a thin envelope called the egg covering. This covering is three-layered and has a fibrillar structure. It was less thick when surrounding older embryos. After PAS-AB the covering stained very weakly; PAT-Ag Prot gave negative results. Lectin-cytochemistry showed that the inner and outer layer contain β-D-galactose; bromobimane-fluorescence revealed the occurrence of thiol groups and disulfide bonds. With different tracers it was demonstrated that the covering is permeable to water, lead and phosphate ions, amino acids (L-(35s) methionine), proteins (horseradish peroxidase, 40,000 Da) and probably also to substances with a higher molecular weight as indicated by some preliminary experiments with FITC-conjugated dextrans. SDS gel electrophoresis revealed several proteins in the albumen, which appear to be (at least partly) identical with those in the fluid of the brood pouch. Presumably the high permeability of the egg covering permits transfer of proteins from the brood pouch to the embryo. However, previous experiments suggested that possible feeding of the young by the mother seems not to be essential for normal development. Current evidence suggests, therefore, thatL. saxatilismay be regarded as lecithotroph viviparous.