479 Rhumbler, Das Protoplasma als physikalisches System, Jena, p. 591, 1914; also in Arch. f. Entwickelungsmech. VII, pp. 279ã335, 1898.
480 Verworn, Psycho-physiologische Protisten-Studien, Jena, 1889 (219 pp.).
481 Leidy, J., Fresh-water Rhizopods of N. America, 1879, p. 262, pl. xli, figs. 11, 12.
482 Carnoy, Biologie Cellulaire, p. 244, fig. 108; cf. Dreyer, op. cit. 1892, fig. 185.
483 In all these latter cases we recognise a relation to, or extension of, the principle of Plateauãs bourrelet, or van der Mensbruggheãs masse annulaire, of which we have already spoken (p. 297).
484 Apart from the fact that the apex of each pyramid is interrupted, or truncated, by the presence of the little central cell, it is also possible that the solid angles are not precisely equivalent to those of Maraldiãs pyramids, owing to the fact that there is a certain amount of distortion, or axial asymmetry, in the Nassellarian system. In other words (to judge from Haeckelãs figures), the tetrahedral symmetry in Nassellaria is not absolutely regular, but has a main axis about which three of the trihedral pyramids are symmetrical, the fourth having its solid angle somewhat diminished.
485 Cf. Faradayãs beautiful experiments, On the Moving Groups of Particles found on Vibrating Elastic Surfaces, etc., Phil. Trans. 1831, p. 299; Researches in Chem. and Phys. 1859, pp. 314ã358.
486 We need not go so far as to suppose that the external layer of cells wholly lacked the power of secreting a skeleton. In many of the Nassellariae figured by Haeckel (for there are many variant forms or species besides that represented here), the skeleton of the partition-walls is very slightly and scantily developed. In such a case, if we imagine its few and scanty strands to be broken away, the central tetrahedral figure would be set free, and would have all the appearance of a complete and independent structure.
487 The ãbourreletã is not only, as Plateau expresses it, a ãsurface of continuity,ã but we also recognise that it tends (so far as material is available for its production) to further lessen the free surface-area. On its relation to vapour-pressure and to the stability of foam, see FitzGeraldãs interesting note in Nature, Feb. 1, 1894 (Works, p. 309).
488 Of the many thousand figures in the hundred and forty plates of this beautifully illustrated book, there is scarcely one which does not depict, now patently, now in pregnant suggestion, some subtle and elegant geometrical configuration.