On the ovum becoming detached the micropyle still remains as an aperture, which probably has the function of admitting the spermatozoa. The shape and form of the micropyle vary greatly. In Anodon and Unio it is a projecting trumpet-shaped structure, which after fertilization becomes shortened and reduced to a mere aperture which is finally stopped up. (Fig. 12.) In other forms it is simply a perforation in the vitelline membrane which is sometimes very large. In a species of Arca, which I had an opportunity of observing at Valparaizo, it was equal to nearly the circumference of the ovum. The eggs of the Lamellibranchiata are not only remarkable in the possession of a micropyle, but in certain peculiarities of the yolk and of the germinal vesicle. In the fresh-water mussels there is usually found in young and medium-sized ova a peculiar lens-shaped body--Keber's corpuscle--which is placed immediately internal to the micropyle. It is probably in some way connected with the nutrition of the ovum, though the fact that it is not always present shews that it cannot be of great importance. A dark body found by von Jhering in the neighbourhood of the germinal vesicle in the ripe ovum of Scrobicularia is probably of a similar nature to Keber's corpuscle. Both bodies may be placed in the same category as the so-called yolk nucleus of the spider's and frog's ova. In all except the youngest ova of Anodon and Unio the germinal spot is composed of two nearly complete spheres united together for a small part of their circumference. (Fig. 12, _gs._) The smaller of these has a higher refractive index than the larger, and often contains a vacuole: the two parts together appear to be the separated components (though not by simple division) of the primitive nucleolus. A nucleolus of this character is not universal amongst Lamellibranchiata, but a similar separation of the constituents of the germinal spot has been found by Flemming in Tichogonia, in which however the more highly refracting body envelopes part of the less highly refracting body in a cap-like fashion. _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.)