(24) Th. von Hessling. "Einige Bemerkungen, etc." _Zeit. f. wiss. Zool._, Bd. V. 1854. (25) H. von Jhering. "Zur Kenntniss d. Eibildung bei d. Muscheln." _Zeit. f. wiss. Zool._, Vol. XXIX. 1877. (26) Keber. _De Introitu Spermatozoorum in ovula_, etc. Königsberg, 1853. (27) Fr. Leydig. "Kleinere Mittheilung etc." _Müller's Archiv_, 1854. _Gasteropoda._ (28) C. SEMPER. "Beiträge z. Anat. u. Physiol. d. Pulmonaten." _Zeit. f. wiss. Zool._, Vol. VIII. 1857. (29) H. Eisig. "Beiträge z. Anat. u. Entwick. d. Pulmonaten." _Zeit. f. wiss. Zool._, Vol. XIX. 1869. (30) Fr. Leydig. "Ueb. Paludina vivipara." _Zeit. f. wiss. Zool._, Vol. II. 1850. _Cephalopoda._ (31) AL. KÖLLIKER. _Entwicklungsgeschichte d. Cephalopoden._ Zurich, 1844. (32) E. R. Lankester. "On the developmental History of the Mollusca." _Phil. Trans._, 1875. _Lamellibranchiata._ The ova of the Lamellibranchiata present several points of interest. They are developed in pouches of the ovary which are lined by a flattened germinal epithelium, or sometimes (?) a syncytium. Some of the cells of this epithelium enlarge and become ova, but remain attached to the walls of their pouches by protoplasmic stalks. Round the ovum there appears in some forms (Anodon, Unio) a delicate vitelline membrane, which is incomplete at the protoplasmic stalk, and is therefore perforated by an aperture which forms the micropyle. (Fig. 12.) As the ovum becomes ripe a large space filled with albuminous fluid becomes established between the ovum and its membrane, but the ovum remains attached to the membrane at the micropyle. In Scrobicularia (von Jhering, No. 25) the membrane round the ovum appears from the first as an albuminous layer, the outermost stratum of which becomes subsequently hardened as the vitelline membrane. In this form also the protoplasmic stalk becomes, in pouches largely filled with ova, extremely long. The ova become eventually detached by the stalk rupturing, and the portion of it which remains attached to the vitelline membrane falling off. The function of the stalk and of the micropyle during the development of the ovum is undoubtedly a nutritive one. In Anodon and Unio yolk granules similar to those deposited in the protoplasm of the ovum are also found in the epithelial cells of the ovarian pouches (Flemming, 22), and there can be but little doubt that they are directly transported from these cells into the ovum. These cells would seem therefore to play much the same part as the yolk-glands of some Turbellarians (Prostomum caledonicum). In Scrobicularia yolk granules are not found in the epithelium of the pouches, but are contained in the dilated disc by which the ovum is attached to the wall of its pouch, as well as in the ovum itself. [FIG. 12. MEDIUM-SIZED OVUM OF ANODONTA COMPLANATA. (After Flemming.) _mp._ micropyle. _gs._ germinal spot.]