Footnote 109: Arch. f. mikr. Anat., Bd. 23. p. 182, 1884. Footnote 110: Born, ‘Biologische Untersuchungen,’ I, Arch. Mikr. Anat., Bd. XXIV. Footnote 111: Roux, ‘Beiträge zum Entwicklungsmechanismus des Embryo,’ 1884. Footnote 112: O. Hertwig, ‘Welchen Einfluss übt die Schwerkraft,’ etc. Jena, 1884. Footnote 113: [Our present knowledge of the development of vegetable ova (including the position of the parts of the embryo) is also in favour of the view that it is not influenced by external causes, such as gravitation and light. It takes place in a manner characteristic of the genus or species, and essentially depends on other causes which are fixed by heredity, see Heinricher ‘Beeinflusst das Licht die Organanlage am Farnembryo?’ in Mittheilungen aus dem Botanischen Institute zu Graz, II. Jena, 1888.—S. S.] Footnote 114: E. van Beneden, ‘Recherches sur la maturation de l’œuf,’ etc., 1883. Footnote 115: M. Nussbaum, ‘Ueber die Veränderung der Geschlechtsprodukte bis zur Eifurchung,’ Arch. Mikr. Anat., 1884. Footnote 116: Eduard Strasburger, ‘Neue Untersuchungen über den Befruchtungsvorgang bei den Phanerogamen als Grundlage für eine Theorie der Zeugung.’ Jena, 1884. [It is now generally admitted that, in the Vascular Cryptogams, as also in Mosses and Liverworts, the bodies of the spermatozoids are formed by the nuclei of the cells from which they arise. Only the cilia which they possess, and which obviously merely serve as locomotive organs, are said to arise from the surrounding cytoplasm. It is therefore in these plants also the nucleus of the male cell which effects the fertilization of the ovum. See Göbel, ‘Outlines of Classification and Special Morphology,’ translated by H. E. F. Garnsey, edited by I. B. Balfour, Oxford, 1887, p. 203, and Douglas H. Campbell, ‘Zur Entwicklungsgeschichte der Spermatozoiden,’ in Berichte d. deutschen bot. Gesellschaft, vol. v (1887), p. 120.—S. S.] Footnote 117: O. Hertwig, ‘Das Problem der Befruchtung und der Isotropie des Eies.’ Jena, 1885. Footnote 118: This opinion was first expressed in my lecture, ‘Ueber die Dauer des Lebens,’ Jena, 1882, translated as the first essay in the present volume. Footnote 119: M. Nussbaum, ‘Sitzungber. der Niederrheinischen Gesellschaft fur Natur- und Heilkunde.’ Dec. 15, 1884. Footnote 120: A. Gruber, ‘Biologisches Centralblatt,’ Bd. IV. No. 23, and V. No. 5. Footnote 121: According to the observations of Nussbaum and van Beneden, the egg of _Ascaris_ departs from the ordinary type, but I think that the latter observer goes too far when he concludes from the form of the nuclear spindle (of which the two halves are inclined to each other at an angle) that we have before us a process entirely different from that of ordinary nuclear division. Footnote 122: Trinchese, ‘I primi momenti dell’ evoluzione nei molluschi,’ Atti Acad. Lyncei (3) vii. 1879, Roma.