Fig. 14. Transverse section through an ovarian egg of Sepia. (Copied from Lankester.)
o.c. outer capsular membrane. i.c. inner capsular membrane with follicular epithelium. b.v. blood-vessels in section between the outer and inner capsular membranes. c. vitellus.
The section shews the folds of the inner capsule with their epithelium, which penetrate into the substance of the ovum for the purpose of supplying it with nourishment.
After the above mode of nutrition has gone on for a certain time a change takes place, and the ridges gradually disappear. This is caused by the epithelial cells passing off from the ridges into the protoplasm of the ovum; and becoming assimilated, after retaining their individuality for a longer or shorter period. When the absorption of the ridges is completed the surface of the ovum assumes a perfectly regular outline. The capsule of the ovum then bursts at the opposite pole to the peduncle, and the ovum falls into the oviduct.
The ova of the Cephalopoda, like those of the Gasteropoda, are quite naked, being without a vitelline membrane or chorion. The egg-capsule which is formed for them in their passage down the oviduct is perforated in Sepia by a micropylar aperture.
Chætopoda.
(33) Ed. Claparède. “Les Annelides Chætopodes d. Golfe de Naples.” Mém. d. l. Sociét. phys. et d’hist. nat. de Genève 1868‑9 and 1870.
(34) E. Ehlers. Die Borstenwürmer nach system. und anat. Untersuchungen. Leipzig, 1864‑68.
(35) E. Selenka. “Das Gefäss-System d. Aphrodite aculeata.” Niederländisches Archiv f. Zool., Vol. II. 1873.
The ova of the Chætopoda are in most cases developed from the special tracts of the epithelial cells lining parts of the body cavity, which constitute a germinal epithelium ([fig. 15]). Very frequently (Aphrodite, Arenicola), as is so common in other types, these tracts of germinal cells surround the blood-vessels. In some cases the germinal epithelium thickens to form a compact organ, for which the outermost cells may form a more or less definite membranous covering (Oligochæta, etc.). The ova are formed by the enlargement, accompanied by other changes, of these germinal cells. During their early development the ova are frequently surrounded by a special capsule, which is often stalked, and provided at its attachment with a large micropylar aperture. In Aphrodite and Polynoe this arrangement, which is clearly connected with the nutrition of the ovum, is very easily seen. The ovum is dehisced into the body cavity by the bursting of its capsule or the rupture of the stalk. The capsule is always eventually thrown off; but a vitelline membrane is frequently developed after the detachment of the ovum into the body cavity. The vitelline membrane of Spio and other Polychæta is provided with an equatorial ring of ampulliform vesicles.