_ov._ ovum. _fe._ flattened follicular epithelium.] The ova originate from certain cells (germinal cells) in the peritoneal investment of the ventral vessel, overlying the nervous cord. These cells, which are well marked off from the surrounding flattened peritoneal elements, increase in number by division, and form small masses surrounded by a follicle of peritoneal cells, and attached by a stalk to the peritoneum. The central cell of each mass grows larger than the rest, which arrange themselves in a columnar fashion round it; it is not, however, destined to become the ovum. On the contrary certain of the other cells adjoining the stalk grow larger, and finally one of these becomes distinguished as the ovum by its greater size and the character of its nucleus. The remainder of the larger cells become of the same size as their neighbours. The ovum now becomes more or less separate from the mass of germinal cells, rapidly grows in size, and soon forms the most considerable constituent of the follicle (fig. 16, _ov_). The remaining germinal cells are quite passive, and though, with the exception of the central cell, they do not appear to atrophy, they soon constitute a relatively small prominence on the surface of the ovum. By the rupture of the stalk the whole follicle becomes eventually detached, and the further development of the ovum takes place in the body cavity. A vitelline membrane is formed, and eventually the ovum is taken into the oviduct (segmental organ). At this time or slightly before, the follicle cells together with the germinal mass, which throughout exhibits no signs of atrophy, become thrown off, and the ovum is left invested in its vitelline membrane. NEMATODA. (44) Ed. Claparède. _De la formation et de la fécondation des oeufs chez les Vers Nématodes._ Genève, 1859. (45) R. Leuckart. _Die menschlichen Parasiten._ (46) H. Munk. "Ueb. Ei- u. Samenbildung u. Befruchtung b. d. Nematoden." _Zeit. f. wiss. Zool._, Vol. IX. 1858. (47) H. Nelson. "On the reproduction of Ascaris mystax, etc." _Phil. Trans._ 1852. (48) A. Schneider. _Monographie d. Nematoden._ Berlin, 1866.