(42) Z. Gerbe. "_Recherches sur la segmentation de la cicatrule et la formation des produits adventifs de l'oeuf des Plagiostomes et particulièrement des Raies._" Vide also _Journal de l'Anatomie et de la Physiologie_, 1872. (43) W. His. "Ueb. d. Bildung v. Haifischenembryonen." _Zeit. für Anat. u. Entwick._, Vol. II. 1877. (44) A. Kowalevsky. "Development of Acanthias vulgaris and Mustelus lævis." (_Russian._) _Transactions of the Kiew Society of Naturalists_, Vol. I. 1870. (45) R. Leuckart. "Ueber die allmählige Bildung d. Körpergestalt bei d. Rochen." _Zeit. f. wiss. Zool._, Bd. II., p. 258. (46) Fr. Leydig. _Rochen u. Haie._ Leipzig, 1852. (47) A. W. Malm. "Bidrag till kännedom om utvecklingen af Rajæ." _Kongl. vetenskaps akademiens förhandlingar._ Stockholm, 1876. (48) Joh. Müller. _Glatter Haie des Aristoteles und über die Verschiedenheiten unter den Haifischen und Rochen in der Entwicklung des Eies._ Berlin, 1840. (49) S. L. Schenk. "Die Eier von Raja quadrimaculata innerhalb der Eileiter." _Sitz. der k. Akad. Wien_, Vol. LXXIII. 1873. (50) Alex. Schultz. "Zur Entwicklungsgeschichte des Selachiereies." _Archiv für micro. Anat._, Vol. XI. 1875. (51) Alex. Schultz. "Beitrag zur Entwicklungsgeschichte d. Knorpelfische." _Archiv für micro. Anat._, Vol. XIII. 1877. (52) C. Semper. "Die Stammesverwandschaft d. Wirbelthiere u. Wirbellosen." _Arbeit. a. d. zool.-zoot. Instit. Würzburg_, Vol. II. 1875. (53) C. Semper. "Das Urogenitalsystem d. Plagiostomen, etc." _Arbeit. a. d. zool.-zoot. Instit. Würzburg_, Vol. II. 1875. (54) Wyman. "Observations on the Development of Raja batis." _Memoirs of the American Academy of Arts and Sciences_, Vol. IX. 1864. CHAPTER IV. TELEOSTEI. The majority of the Teleostei deposit their eggs before impregnation, but some forms are viviparous, _e.g._ Blennius viviparus. Not a few carry their eggs about; but this operation is with a few exceptions performed by the male. In Syngnathus the eggs are carried in a brood-pouch of the male situated behind the anus. Amongst the Siluroids the male sometimes carries the eggs in the throat above the gill clefts. Ostegeniosus militaris, Arius falcarius, and Arius fissus have this peculiar habit. The ovum when laid is usually invested in the zona radiata only, though a vitelline membrane is sometimes present in addition, _e.g._ in the Herring. It is in most cases formed of a central yolk mass, which may either be composed of a single large vitelline sphere, or of distinct yolk spherules. The yolk mass is usually invested by a granular protoplasmic layer, which is especially thickened at one pole to form the germinal disc. In the Herring's ovum the germinal disc is formed, as in many Crustacea, at impregnation; the protoplasm which was previously diffused through the egg becoming aggregated at the germinal pole and round the periphery.