Footnote 123: M. Nussbaum, ‘Archiv für Mikroskopische Anatomie,’ Bd. XVIII und XXIII. Footnote 124: Valaoritis, ‘Die Genesis des Thier-Eies.’ Leipzig, 1882. Footnote 125: Kölliker, ‘Die Bedeutung der Zellkerne,’ etc.; Zeitschr. f. wiss. Zool. Bd. XLII. Footnote 126: ‘Compt. rend.’ Tom. LIV. p. 150. Footnote 127: ‘Entwicklung der Dipteren.’ Leipzig, 1864. Footnote 128: ‘Zeitschr. f. wiss. Zool.’ Bd. XVI. p. 389 (1866). Footnote 129: ‘Compt. rend.’ Nov. 13, 1882. Footnote 130: Grobben, ‘Arbeiten d. Wien. Zool. Instituts,’ Bd. II. p. 203. Footnote 131: Bütschli, ‘Zeitschrift f. wiss. Zool.’ Bd. XXIII. p. 409. Footnote 132: ‘Science,’ vol. iv. No. 90, 1884. Footnote 133: Among unicellular organisms, encysted individuals are often called germs. They sometimes differ from the adult organism in their smaller size and simpler structure (_Gregarinidae_), but they represent the same morphological stage of individuality. Footnote 134: Compare Bütschli in Bronn’s ‘Klassen und Ordnungen des Thierreichs,’ Bd. I. p. 777. Footnote 135: Gustav Jäger, ‘Lehrbuch der Allgemeinen Zoologie,’ Leipzig, 1878; II. Abtheilung. Probably on account of the extravagant and superficial speculations of the author, the valuable ideas contained in his book have been generally overlooked. It is only lately that I have become aware of Jäger’s above-mentioned hypothesis. M. Nussbaum seems to have also arrived at the same conclusion quite independently of Jäger. The latter has not attempted to work out his hypothesis with any degree of completeness. The above-mentioned observations are followed immediately by quite valueless considerations, as, for instance, that the ontogenetic and phyletic groups are in concentric ratio! The author might as well speak of a quadrangular or triangular ratio! Footnote 136: [Facts of the same kind are also known in the Vascular Cryptogams, Muscineae, Characeae, Florideae, etc.—S. S.] Footnote 137: Weismann, ‘Die Entstehung der Sexualzellen bei den Hydromedusen.’ Jena, 1883. Footnote 138: [I adopt this term, suggested by E. Ray Lankester and G. C. Bourne, as the name of the supporting lamina of Coelenterata. See ‘Quart. Journ. Microsc. Sci.’ Jan. 1887, p. 28.—E. B. P.] Footnote 139: Dr. Clemens Hartlaub, ‘Ueber die Entstehung der Sexualzellen bei Obelia.’ Freiburg, Inaugural Dissertation: see also ‘Zeitschrift für wissenschaftliche Zoologie.’ Bd. XLI. 1884. Footnote 140: English translation, by H. Marshall Ward. Oxford, 1887, Clarendon Press. Footnote 141: [Such gland-cells are known in both animals and plants. See W. Gardiner and Tokutaro Ito, On the structure of the mucilage-secreting cells of _Blechnum occidentale_ L., and _Osmunda regalis_ L., ‘Annals of Botany,’ vol. i. p. 49.—S. S.] Footnote 142: