[167]. l. c., p. 150.
[168]. Carl Düsing, ‘Die Regulirung des Geschlechtsverhältnisses.’ Jena. 1884.
[169]. I intend to publish these experiments elsewhere in connexion with other observations.
[170]. Weismann, ‘Daphniden,’ Abhandlung, VII. p. 329; Herbert Spencer, ‘The Principles of Biology,’ 1864, vol. i. pp. 229, 230.
[171]. The same fact has since been ascertained in species belonging to several groups of animal.
[172]. Brooks, ‘The Law of Heredity.’ Baltimore, 1883, p. 73.
[173]. ‘Zeitschrift für wissenschaftliche Zoologie,’ Bd. XXXIII. p. 107. 1873.
[174]. Valaoritis, l. c., p. 6.
[175]. I quote from Falkenberg, in Schenk’s ‘Handbuch der Botanik,’ Bd. II. p. 219. He further states that these are the only instances hitherto known in which undoubted male cells have proved to be capable of further development when they have been unable to exercise their powers of fertilization. It must be added that the two kinds of germ-cells do not differ in appearance, but only in behaviour; the female germ-cells becoming fixed, and withdrawing one of their two flagella, while the male cells continue to swarm. But even this slight degree of differentiation requires the supposition of internal molecular differentiation.