Its development at this point appears to be due to the fact that the follicle is here incomplete; so that the cuticular membrane deposited by it is also incomplete. A true vitelline membrane can in many instances be demonstrated (Donacia, etc.). ARANEINA. (55) Victor Carus. "Ueb. d. Entwick. d. Spinneneies." _Zeit. f. wiss. Zool._, Vol. II. 1850. (56) v. Wittich. "Die Entstehung d. Arachnideneies im Eierstock, etc." Müller's _Archiv._ 1849. [Conf. Leydig, Balbiani, Ludwig (No. 4), etc.] The ova of many Araneina are remarkable for the presence in the ovum of the so-called yolk-nucleus. The ova develop from the epithelial cells lining the ovarian sack. Certain of these cells grow large and project outwards, invested by the structureless membrane of the ovarian wall. The stalks of projections so formed are turned towards the lumen of the ovary, and are plugged with the epithelial cells which line the ovarian sack. When ripe, the ova pass from their sacks into the cavity of the ovary. The yolk-nucleus, which appears very early, is a solid body present in the protoplasm of the ovum. It is not found in all genera of Araneina. At its full development it exhibits in the fresh condition a granular structure, but very soon shews an irregularly concentric stratification which becomes more marked on the addition of reagents. According to Balbiani this stratification is confined to the superficial layers, while internally there is a body with all the characters of a cell. The yolk-nucleus is still found in the nearly ripe ovum, though it always disappears before development commences. It is probably connected with the nutrition of the ovum, though nothing is certainly known about its function. CRUSTACEA. (57) Aug. Weismann. "Ueb. d. Bildung von Wintereiern bei Leptodora hyalina." _Zeit. f. wiss. Zool._, Vol. XXVII. 1876. [For general literature _vide_ Ludwig No. 4 and Ed. van Beneden, No. 1.] Amongst the many interesting observations on the Crustacean ova I will only allude to those of Weismann on the ova of Leptodora, a well-known Cladoceran form. The phenomena of the development of the ova in this form present a close analogy with those in Insects.