12. Hertwig, Oscar. Ibid. Morphologisches Jahrbuch, Bd.III, Heft. 1.
13. Hertwig, Oscar. “Weitere Beiträge, &c.” Morphologisches Jahrbuch, Bd. III, Heft 3.
14. Kleinenberg. Hydra. Leipzig, 1872.
15. Oellacher, J. “Beiträge zur Geschichte des Keimbläschens im Wirbelthiereie.” Archiv f. micr. Anat., Bd.VIII.
16. Selenka. Befruchtung u. Theilung des Eies von Toxopneustes variegatus (Vorläufige Mittheilung). Erlangen, 1877.
17. Strasburger. Ueber Zellbildung u. Zelltheilung. Jena, 1876.
18. Strasburger. Ueber Befruchtung u. Zelltheilung. Jena, 1878.
19. R. Greeff. “Ueb. d. Bau u. d. Entwicklung d. Echinodermen.” Sitzun. der Gesellschaft z. Beförderung d. gesammten Naturwiss. z. Marburg, No. 5. 1876.
Postscript.—Two important memoirs have appeared since this paper was in type. One of these by Hertwig, Morphologisches Jahrbuch, Bd. IV, contains a full account with illustrations of what was briefly narrated in his previous paper (13); the other by Calberla, “Der Befruchtungsvorgang beim Ei von Petromyzon Planeri,” Zeit. für wiss. Zool., Bd. XXX, shews that the superficial layer of the egg is formed by a coating of protoplasm free from yolk-spheres, which at one part is continued inwards as a column, and contains the germinal vesicle. The surface of this column is in contact with a micropyle in the egg-membrane. Impregnation is effected by the entrance of the head of a single spermatozoon (the tail remaining outside) through the micropyle, and then along the column of clear protoplasm to the female pronucleus.
[362] From the Quarterly Journal of Microscopical Science, April, 1878.