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. XIV. 1871.