The most anterior nerve is the fifth (fig. 14, V), of which two branches are at this stage developed. The natural interpretation of its present condition is, that it is equivalent to two nerves, but the absence of relation in its branches to any visceral clefts renders it more difficult to determine the morphology of the fifth nerve than of the other nerves. The front branch of the two is the ophthalmic branch of the adult, and the hind branch the inferior maxillary branch. The latter branch subsequently gives off low down, _i.e._ near its distal extremity, another branch, the superior maxillary branch. In its embryonic condition this latter branch does not appear like a third branch of the fifth, equivalent to the seventh or the glosso-pharyngeal nerves, but rather resembles the branch of the seventh nerve which passes to the arch in front, which also is present in all the other cranial nerves. _Modes of Preparation._ Before concluding I will say one or two words as to my modes of preparation. I have used picric and chromic acids, both applied in the usual way; but for the early stages I have found osmic acid by far the most useful reagent. I placed the object to be hardened, in osmic acid (half per cent.) for two hours and a half, and then for twenty four in absolute alcohol. I then embedded and cut sections of it in the usual way, without staining further. I found it advantageous to cut sections of these embryos immediately after hardening, since if kept for long in the absolute alcohol the osmic acid specimens are apt to become brittle. LIST OF WORKS REFERRED TO. Gegenbaur. _Anat. der Wirbelthiere_, III Heft, Leipzig, 1873. A. Götte. _Archiv. für Micr. Anat._, Vol. X. 1873. "Der Keim der Forelleneies," _Archiv. für. Micr. Anat._, Vol. IX. 1873. "Untersuchung über die Entwicklung der Bombinator igneus," _Archiv. für Micr. Anat._, Vol. V. 1869. "Kurze Mittheilungen aus der Entwicklungsgeschichte der Unke," _Archiv. für Micr. Anat._, Vol. IX. 1873. Kupffer. _Archiv. für Micr. Anat._, Vol. II. 1866, p. 473. Ibid. Vol. IV. 1868, p. 209. Kowalevsky. "Entwicklungsgeschichte der Holothurien," _Mémoires de l'Acad. Impér. des Sciences de St Petersbourg_, vii ser. Vol. XI. 1867. Kowalevsky, Owsjannikow, und Wagner. "Entwicklung der Störe," _Bulletin der K. Acad. St Petersbourg_, Vol. XIV. 1873. Kowalevsky. "Embryologische Studien an Würmern und Arthropoden," _Mémoirs de l'Acad. Impér. des Sciences de St Petersbourg_, Vol. XVI. 1871. E. Ray Lankester. _Annals and Mag. of Nat. History_, Vol. XI. 1873, p. 81. W. Müller. "Ueber die Persistenz der Urniere bei Myxine Glutinosa," _Jenaische Zeitschrift_, Vol. VII. 1873. Oellacher. _Zeitschrift für Wiss. Zoologie_, Vol. XXIII. 1873. Owsjannikow. "Entwicklung der Coregonus," _Bul. der K. Akad. St Petersbourg_, Vol. XIX. Romiti. _Archiv. für Micr. Anat._, Vol. IX. 1873. Waldeyer. _Eierstock u. Eie._