2. Considerable changes are effected in the gill pouches, and, according to Schneider, whose statements must however be received with some caution, the branchial sack becomes detached posteriorly from the oesophagus, the oesophagus then sends forwards a prolongation above the branchial sack which is at first solid. This prolongation forms the anterior part of the oesophagus of the adult, and joins the primitive oral cavity at the velum. The so-called bronchus of the adult is thus the whole branchial region of the Ammocoetes, and the anterior part of the oesophagus of the adult is an entirely new formation. 3. The posterior part of the alimentary tract of the Ammocoetes undergoes partial atrophy. The gall-bladder of the liver is absorbed; and the liver itself ceases to communicate with the intestine. 4. The eye undergoes important changes in that it travels to the surface, and acquires all the characters of the normal vertebrate eye. 5. The brain becomes relatively larger but more compact, and the optic lobes (corpora bigemina) become more distinct. 6. The pericardial cavity becomes completely separated from the body cavity, and a distinct pericardium is formed. 7. The mesonephros of the larva disappears, and a fresh posterior part is formed. Myxine. The ovum of Myxine when ready to be laid is inclosed, as shewn by Allen Thomson[32], in an oval horny shell in many respects similar to that of Elasmobranchii; from its ends there project a number of trumpet-shaped tubular processes, which no doubt serve to attach it to marine objects. No observations have been made on the development. [32] _Cyclopædia of Anat. and Phys._ Article 'Ovum.' BIBLIOGRAPHY. (77) E. Calberla. "Der Befruchtungsvorgang beim Petromyzon Planeri." _Zeit. f. wiss. Zool._, Vol. XXX. 1877. (78) E. Calberla. "Ueb. d. Entwicklung d. Medullarrohres u. d. Chorda dorsalis d. Teleostier u. d. Petromyzonten." _Morpholog. Jahrbuch_, Vol. III. 1877. (79) C. Kupffer u. B. Benecke. _Der Vorgang d. Befruchtung am Ei d. Neunaugen._ Königsberg, 1878. (80) Aug. Müller. "Ueber die Entwicklung d. Neunaugen." Müller's _Archiv_, 1856. (81) Aug. Müller. _Beobachtungen üb. d. Befruchtungserscheinungen im Ei d. Neunaugen._ Königsberg, 1864. (82) W. Müller. "Das Urogenitalsystem d. Amphioxus u. d. Cyclostomen." _Jenaische Zeitschrift_, Vol. IX. 1875. (83) Ph. Owsjannikoff. "Die Entwick. von d. Flussneunaugen." Vorläuf. Mittheilung. _Mélanges Biologiques tirés du Bulletin de l'Acad. Imp. St Pétersbourg_, Vol. VII. 1870. (84) Ph. Owsjannikoff. _On the development of Petromyzon fluviatilis_ (Russian). (85) Anton Schneider. _Beiträge z. vergleich. Anat. u. Entwick. d. Wirbelthiere._ Quarto. Berlin, 1879. (86) M. S. Schultze. "Die Entwickl. v. Petromyzon Planeri." Gekrönte Preisschrift. Haarlem, 1856. (87) W. B. Scott. "Vorläufige Mittheilung üb. d. Entwicklungsgeschichte d. Petromyzonten." _Zoologischer Anzeiger_, Nos. 63 and 64. III. Jahrg. 1880.