The most probable view of the nature of the vitellarium is that of Gegenbaur, Hallez, etc., according to which it is to be regarded as a specially modified part of the ovarian tube. On this view the nature and function of the yolk cells admit of a fairly simple explanation. They are to be regarded as primary germinal cells like those in the ovaries of Hydra, Tubularia, etc., which do not become converted into ova. Like these cells they may in some instances, Macrostomum, Prostomum, etc., serve directly in the nutrition of the ovum. In other cases they retain their independence and serve for the late nutrition of the embryo. In both instances they retain the faculty, normally possessed by ova, of forming yolk particles in their protoplasm. ECHINODERMATA. (15) C. K. Hoffmann. "Zur Anatomie d. Echiniden u. Spatangen." _Niederländisch. Archiv f. Zoologie_, Vol. I. 1871. (16) C. K. Hoffmann. "Zur Anatomie d. Asteriden." _Niederländisch. Archiv f. Zoologie_, Vol. II. 1873. (17) H. Ludwig. "Beiträge zur Anat. d. Crinoiden." _Zeit. f. wiss. Zool._, Vol. XXVIII. 1877. (18) Joh. Müller. "Ueber d. Canal in d. Eiern d. Holothurien." Müller's _Archiv_, 1854. (19) C. Semper. _Holothurien._ Leipzig, 1868. (20) E. Selenka. _Befruchtung d. Eies v. Toxopneustes variegatus_, 1878. [_Vide_ also Ludwig (No. 4), etc.] The eggs of the Echinodermata present in their development certain points of interest. The ovaries themselves are usually surrounded by a special vascular dilatation. In the Asteroidea, the Echinoidea, and the Holothuroidea the organs have the form of sacks; specially surrounded in the two former groups, and probably the latter, by a vascular sinus formed as a dilatation of one of the generative vessels. In the Crinoids they have the form of a hollow rachis completely surrounded by a blood-vessel. (Fig. 11, _b._) The proximity of the ovaries (generative organs) to the vascular system in these forms has clearly the same physiological significance as the proximity of the ovaries (generative organs) to the radial vessels in the Coelenterata. In the Asteroidea, the Echinoidea and the Holothuroidea the ovaries have the form of sacks lined by an epithelium of germinal cells, and the ova are formed by the enlargement of these cells, which, when they have reached a certain size, become detached from the walls, and fall into the cavity of the ovarian sack. In Toxopneustes (Selenka) and very probably in other forms only a few of the epithelial cells undergo conversion into ova: the remainder undergo repeated division, and, as in so many other cases, are eventually employed in the nutrition of the true ova. In the nearly ripe ova of Asterias Fol has described a flattened follicular epithelium the origin of which is unknown. [FIG. 10. OVUM OF TOXOPNEUSTES VARIEGATUS WITH THE PSEUDOPODIA-LIKE PROJECTIONS OF THE PROTOPLASM PENETRATING THE ZONA RADIATA (_zr_). (After Selenka.)]