[20]

See Hartog, "On Multiple Cell-division, as compared with Bi-partition as Herbert Spencer's limit of growth," in Rep. Brit. Ass. 1896, p. 833; "On a Peptic Zymase in Young Embryos," ibid. 1900, p. 786; "Some Problems of Reproduction," ii. Quart. Journ. Micr. Sci. xlvii. 1904, p. 583.

[21]

"On the Digestive Ferment of a large Protozoon." Rep. Brit. Ass. 1893, p. 801.

[22]

See for studies of the movements of Protoplasm, Berthold, Protoplasmamechanik (1886); Bütschli, Investigations on Microscopic Foams and on Protoplasm, English ed. 1894; Verworn, General Physiology, 1899; Le Dantec, La Matière Vivante, 1893?; and Jensen, "Unters. ueb. Protoplasmamechanik," in Arch. Ges. Phys. lxxxvii. 1901, p. 361; Davenport, Experimental Morphology, i. 1897; H. S. Jennings, Contr. etc. 1904.

[23]

The terms "expansion" and "contraction" refer only to the superficial area: it is very doubtful whether the volume alters during these changes.

[24]

For discussions on the mechanism of ciliary action, see Schäfer, Anat. Anz. xxiv. 1904, p. 497, xxvi. 1905, p. 517; Schuberg, Arch. Protist. vi. 1905, p. 85.