They first appear as small highly refracting granules; in these a cavity is formed which is at first central but is eventually pushed to one side by the formation of a conical projection from the wall of the vesicle. After the growth of the ovum is completed the amoeboid processes gradually withdraw themselves, and the ovum assumes a spherical form; still however continuing to be invested by the remaining cells of the ovary. It is important to notice that the egg of Hydra retains throughout its whole development the characters of a single cell, and that the pseudocells and other structures which make their appearance in it are not derived from without, and supply not the slightest ground for regarding the ovum as a structure compounded of more than one cell. The development of the ova of the Tubularidæ, which has been supposed by many investigators to present very special peculiarities, takes place on essentially the same type as that of Hydra, but the germinal vesicle remains permanently very small and difficult to observe. The mode of nutrition of the ovum may be very instructively studied in this type. The process is one of actual feeding, much as an Amoeba might feed on other organisms. Adjoining one of the large ova of the ovary there may be seen a number of small germinal cells. (Fig. 3.) The boundary between these cells and the ovum is indistinct. Just beyond the edge of the ovum the small cells have begun to undergo retrogressive changes; while at a little distance from the ovum they are quite normal (_g.c._)[20]. [20] The above description of the ova of the Tubularidæ is founded on sections of the gonophores of Tubularia mesembryanthemum. Dr Kleinenberg informs me however that the absence of a distinct boundary between the germinal cells and the ovum is not usual. PLATYELMINTHES. (10) P. Hallez. _Contributions à l'Histoire naturelle des Turbellariés._ Lille, 1879. (11) S. Max Schultze. _Beiträge z. Naturgeschichte d. Turbellarien._ Greifswald, 1851. (12) C. Th. von Siebold. "Helminthologische Beiträge." Müller's _Archiv_, 1836. (13) C. Th. von Siebold. _Lehrbuch d. vergleich. Anat. d. wirbellosen Thiere._ Berlin, 1848. (14) E. Zeller. "Weitere Beiträge z. Kenntniss d. Polystomen." _Zeit. f. wiss. Zool._, Bd. XXVII. 1876. [_Vide_ also Ed. van Beneden] (No. 1).