(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. [FIG. 15. A PARAPODIUM OF TOMOPTERIS. (From Gegenbaur.) _o._ Collection of germinal epithelial cells lining the body cavity.] DISCOPHORA. (36) H. Dorner. "Ueber d. Gattung Branchiobdella." _Zeit. f. wiss. Zool._, Vol. XV. 1865. (37) R. Leuckart. _Die menschlichen Parasiten._ (38) Fr. Leydig. "Zur Anatomie v. Piscicola geometrica, etc." _Zeit. f. wiss. Zool._, Vol. I. 1849. (39) C. O. Whitman. "Embryology of Clepsine." _Quart. J. of Micr. Sci._, Vol. XVIII. 1878. The ovary of the Discophora is formed of a mass of cells enveloped in a membranous sack. In Branchiobdella there is placed in the central axis of these cells a column of nucleated protoplasm from which the cells themselves are budded off. The development of the ovum takes place by the enlargement, etc. of one of the peripheral cells, which eventually bursts the wall of the sack and is freely dehisced into the body cavity. In most other Leeches (except Piscicola and its allies) there is found a more specialized arrangement of the same nature as in Branchiobdella. There are one or more coiled egg-strings which lie freely in a delicate sack continuous with the oviduct. Each egg-string is formed of a central rachis and of a peripheral layer of cells[22]. The ova are formed by the enlargement of the peripheral cells accompanied by a deposition of food-yolk. Food-yolk appears to be formed in the rachis even more energetically than in the protoplasm of the ova. When ripe the ova fall into the ovarian sack.