All the membranes which surround the ovum may be provided with a special aperture known as the micropyle. A micropyle is by no means found in the majority of types, and there is no homology between the various apertures so named. Micropyles have two functions, either (1) to assist in the nutrition of the ovum during its development, or (2) to permit the entrance of the spermatozoa. The two functions may in some cases coexist. Micropyles of the first class are developed at the point of attachment of the ovum to the wall of the ovary or to its follicle. Good examples of this kind of micropyle are afforded by the Lamellibranchiata (fig. 12), Holothuria, and many Annelida (Polynoe, etc.). The micropyle of the Lamellibranchiata (p. 37) probably serves also to admit the spermatozoa. The second type of micropyle is found in many Insecta, Teleostei, etc. GENERAL BIBLIOGRAPHY OF THE OVUM. (1) Ed. van Beneden. "Recherches sur la composition et la signification de l'oeuf," etc. _Mém. cour. d. l'Acad. roy. des Sciences de Belgique_, Vol. XXXIV. 1870. (2) R. Leuckart. Artikel "Zeugung," R. Wagner's _Handwörterbuch d. Physiologie_, Vol. IV. 1853. (3) Fr. Leydig. "Die Dotterfurchung nach ihrem Vorkommen in d. Thierwelt u. n. ihrer Bedeutung." _Oken, Isis_, 1848. (4) Ludwig. "Ueber d. Eibildung im Thierreiche." _Arbeiten a. d. zool.-zoot. Institut Würzburg_, Vol. I. 1874[15]. (5) Allen Thomson. Article "Ovum" in Todd's _Cyclopædia of Anatomy and Physiology_, Vol. V. 1859. (6) W. Waldeyer. _Eierstock u. Ei._ Leipzig, 1870. [15] A very complete and critical account of the literature is contained in this paper. _Special History of the Ovum in different types._ COELENTERATA. (7) Ed. van Beneden. "De la distinction originelle d. testicule et de l'ovaire." _Bull. Acad. roy. Belgique_, 3e série, Vol. XXXVII. 1874. (8) R. and O. Hertwig. _Der Organismus d. Medusen._ Jena, 1878. (9) N. Kleinenberg. _Hydra._ Leipzig, 1872. Amongst the Coelenterata the ova are developed in imperfectly specialized organs, which are situated in various parts of the body, for the most part in the space between the epiblast and the hypoblast. In Hydra the locality where the ova are developed only becomes specialized at the time when an ovum is about to be formed. At one or more points the interstitial cells of the epiblast increase in number and form a protuberance of germinal cells, which may be called the ovary. In this ovary a single ovum is formed by the special growth of one cell. (Kleinenberg, No. 9.) In the free and attached gonophores of Hydrozoa, the ova appear either around the walls of the stomach, or the radial canals, or around other parts of the gastro-vascular canals. [FIG. 8. RIPE OVUM OF EPIBULIA AURANTIACA. THE GERMINAL VESICLE HAS BECOME INVISIBLE WITHOUT REAGENTS. Copied from Metschnikoff, "Entwicklung der Siphonophoren." _Zeitschrift f. wiss. Zool._, Vol. XXIV. 1874.