[22] The rachis is stated by Whitman (No. 39), and other observers to be formed of nucleated protoplasm, but further investigations on this point are still required. In Piscicola the development of the ovum is somewhat peculiar but resembles in certain respects that of Bonellia (p. 45). The ova are developed from the primitive germinal cells which fill up the ovarian sack. The nuclei in these cells increase in number, and a nucleated peripheral layer of each cell becomes separated from the central part, which also contains nuclei. This latter part next divides into numerous cells, of which one eventually forms the ovum, and the remainder constitute a mass of cells adjoining it as in Bonellia (fig. 16). This mass of cells eventually disappears, and is probably employed in the nutrition of the ovum. The ovaries of the Leech appear to belong to the tubular type in that the ova are not formed from part of the epithelium lining the body cavity; but if, as seems probable, the true affinities of the Leeches are with the Chætopoda, the investment of the ovaries must be of a secondary nature. It should be noted that the ova are not, as in the ordinary tubular ovary, developed from the epithelium lining the ovarian tube. GEPHYREA. (40) Keferstein u. Ehlers. _Zoologische Beiträge._ Leipzig, 1861. (41) C. Semper. _Holothurien_, 1868, p. 145. (42) J. W. Spengel. "Beiträge z. Kenntniss d. Gephyreen." _Beiträge a. d. zool. Station z. Neapel_, Vol. I. 1879. (43) J. W. Spengel. "Anatomische Mittheilungen üb. Gephyreen." _Tagebl. d. Naturf. Vers._ München, 1877. In the Gephyrea, as in the Chætopoda, the ova are developed from the lining cells of the peritoneum and frequently from the cells surrounding parts of the vascular system (Bonellia, Thalassema). In many cases (Sipunculus, Phascolosoma, Echiurus) the main growth of the ovum takes place after it has been dehisced into the body cavity. In Sipunculus the ova in the body cavity are surrounded by a follicle which is thrown off before they become ripe. Brandt denies the existence of this follicle or rather its cellular nature. Spengel's (43) observations are conclusive in favour of the correctness of the original interpretation Of Keferstein and Ehlers. The follicles would seem to be formed after the ova have become free. In Phascolosoma there is no follicle (Semper, Spengel). In both Phascolosoma and Sipunculus a vitelline membrane with radial pores--_zona radiata_--is formed, and in Phascolosoma the external part of this is separated off as a structureless vitelline membrane. The formation of both these membranes from the protoplasm of the ovum is rendered certain in the latter case by the absence of a follicular epithelium. Some interesting observations on the growth and origin of the ovum in Bonellia have been made by Spengel. [FIG. 16. FOLLICLE OF BONELLIA AT A MEDIUM STAGE OF DEVELOPMENT. (After Spengel.)