Bütschli, ‘Gedanken über die morphologische Bedeutung der sogenannten Richtungskörperchen,’ Biolog. Centralblatt, Bd. VI. p. 5, 1884. Footnote 155: F. M. Balfour, ‘Comparative Embryology,’ vol. i. p. 63. Footnote 156: The formation of a polar body in parthenogenetic eggs has now been proved: see note at the end of this Essay; see also Essay VI.—A. W., 1888. Footnote 157: R. Leuckart,—article ‘Zeugung,’ in R. Wagner’s ‘Handwörterbuch der Physiologie,’ 1853, Bd. IV. p. 958. Similar observations were made by Max Schultze. These observations appear however to be erroneous, for Pflüger has since shown that the eggs of frogs never develope if the necessary precautions are taken to prevent the access of any spermatozoa to the water.—A. W., 1888. Footnote 158: Oellacher, ‘Die Veränderungen des unbefruchteten Keims des Hühncheneies. ‘Zeitschrift für wissenschaftliche Zoologie,’ Bd. XXII. p. 181. 1872. Footnote 159: Hensen, ‘Centralblatt,’ 1869, No. 26. Footnote 160: Weismann, ‘Beiträge zur Naturgeschichte der Daphnoiden,’ Leipzig, 1876-79, Abhandlung VII, and ‘Zeitschrift für wissenschaftliche Zoologie,’ Bd. XXXIII. Footnote 161: Weismann, ‘Beiträge zur Kenntniss der ersten Entwicklungsvorgänge im Insectenei,’ Bonn, 1882, p. 106. Footnote 162: W. Roux, ‘Ueber die Bedeutung der Kerntheilungsfiguren.’ Leipzig, 1883. Footnote 163: We now know that the number of loops varies considerably in different species, even when they belong to the same group of animals (e.g. Nematodes).—A.W., 1888. Footnote 164: This expression is used by bee-keepers, for instance by the well-known Baron Berlepsch. Of course, it would be more accurate to say that the queen, seeing the cell of a drone, is stimulated to lay an unfertilized egg, and that, on the other hand, she is stimulated to lay a fertilized egg when she sees the cell of a worker, or that of a queen. Footnote 165: E. Bessels, ‘Die Landois’sche Theorie widerlegt durch das Experiment.’ Zeitschrift für wissenschaftliche Zoologie, Bd. XVIII. p. 124. 1868. Footnote 166: ‘Daphniden,’ Abhandlung, vi. p. 324. Footnote 167: l. c., p. 150. Footnote 168: Carl Düsing, ‘Die Regulirung des Geschlechtsverhältnisses.’ Jena. 1884. Footnote 169: I intend to publish these experiments elsewhere in connexion with other observations. Footnote 170: Weismann, ‘Daphniden,’ Abhandlung, VII. p. 329; Herbert Spencer, ‘The Principles of Biology,’ 1864, vol. i. pp. 229, 230. Footnote 171: The same fact has since been ascertained in species belonging to several groups of animal. Footnote 172: Brooks, ‘The Law of Heredity.’ Baltimore, 1883, p. 73. Footnote 173: ‘Zeitschrift für wissenschaftliche Zoologie,’ Bd. XXXIII. p. 107. 1873. Footnote 174: Valaoritis, l. c., p. 6. Footnote 175: