Fine structure and secretion of the capsule enclosing the embryo in a crab (Carcinus maenas (L.))

Carcinus maenas oocyte is encased, at the end of vitellogenesis phase, by two superposed vitelline envelopes. If the outermost (envelope 1a) is relatively thin, the innermost (envelope 1b) is however five times much thicker. The envelope 1b, secreted mostly by the oocyte, contains proteinaceous material and a substance which strongly binds WGA. The 1b material deposition is completed when the oocyte nuclear apparatus still has the characteristics of a germinal vesicle. After the secretion phase and always at the ovarian level, envelope 1b material undergoes remarkable structural modifications. These last events occur while the oocyte nuclear apparatus is in the first meiotic metaphase stage. Later, after egg-laying, a new material appears below the two superimposed vitelline envelopes 1a and 1b. This new innermost material (envelope 2), observed as a very thick and homogeneous secretion, is secreted by the newly laid egg probably after fecondation. Envelope 2 material is wholly deposited in few hours and arises from vesicles of an oocytic origin. These three superimposed envelopes make up the embryo capsule of which the main part is constituted by envelope 2. This tripartite capsule will stay, surrounding the developmental stages, superimposed to newly secreted successive embryonic envelopes, up to the end of embryogenesis.