Gasteropoda.
The ova of the Gasteropoda are developed, like those of the Lamellibranchiata, from the epithelial cells of the ovarian acini or pouches. In the hermaphrodite forms both ova and spermatozoa are produced in the same pouches ([fig. 13]), some of the epithelial cells becoming ova and others spermatozoa. The ova are usually formed in the wall of the pouch, and the spermatozoa internally (Pulmonata) ([fig. 13] A), or a further differentiation of parts may take place ([fig.13] B). The ova of Gasteropods are exceptional in the fact that a vitelline membrane is rarely or never developed around them. The ovum in its passage to the exterior becomes enclosed in a secretion of the albuminous gland, which hardens externally to form a special membrane.
Fig. 13. Follicles of the hermaphrodite glands of Gasteropoda.
(From Gegenbaur.)
A. Of Helix hortensis. The ova (aa) are developed on the wall of the follicle, and the seminal masses (b) internally.
B. Of Aeolidia. The seminal portion of a follicle is beset peripherally by ovarian saccules (a). c. Common afferent duct.
Cephalopoda.
Lankester (No. [32]) has brought out some very interesting points with reference to the nutrition of the eggs of Sepia during their growth. The eggs develop in connective-tissue pouches which early give rise to a double pedunculated capsule of connective tissue. The cells of the inner layer of this capsule soon assume an epithelial character, and become a definite follicular epithelium, while between the two layers there penetrates a network of vascular channels. The follicular epithelium becomes after the establishment of these vascular channels folded in a most remarkable manner. The folds, which are shewn in section in [fig. 14], ic, project into and nearly completely fill up the body of the ovum. An enormous increase is thus effected in the nutritive surface exposed by the epithelium. Each fold is thoroughly supplied with blood-vessels. The plications of the follicular epithelium give rise to a basket-work tracery on the surface of the ovum. During the stage when the follicular epithelium has the above structure, its cells have a character similar to that of the goblet-cells of a mucous membrane, and pour out their metamorphosed protoplasm into the body of the ovum.