418 THE FORMS OF CELLS [ch. 



of protoplasm covering the shell. As a physical phenomenon the 

 actions involved are by no means fully understood, but surface- 

 tension, diffusion and cohesion play their respective parts therein*. 

 The very perfect cellular patterns obtained by Leduc (to which we 

 shall have occasion to refer in a subsequent chapter) are diffusion 

 patterns on a larger scale, but not essentially different. 



The folded or pleated pattern is doubtless to be explained, in 



a general way, by the shrinkage of a surface-film under certain 



conditions of viscous or frictional restraint. 



A case which (as it seems to me) is closely 



alhed to that of our foraminiferal shells is 



described by Quincke t, who let a film of 



chromatised gelatin or of resin set and harden 



upon a surface of quicksilver, and found that 



the little solid pelHcle had been thrown into 



a pattern of symmetrical folds, as fine as a 



Fig.^ 133. diffraction grating. If the surface thus folded 



or wrinkled be a cyhnder, or any other figure with one principal axis 



* This cellular pattern would seem to be related to the "cohesion figures" 

 described by Tomlinson in various surface-films {Phil. Mag. 1861-70); to the 

 "tesselated structure" on liquid surfaces described by James Thomson in 1882 

 {Collected Papers, p. 136); and (more remotely) to the tourhillons cellulaires of 

 Benard, Ann. de Chimie (7), xxiii, pp. 62-144, 1901; (8), xxiv, pp. 563-566, 1911, 

 Bev. gener. des Sci. xi, p. 1268, 1900; cf. also E. H. Weber, Mikroskopische Beo- 

 bachtungen sehr gesetzmassiger Bewegungen welche die Bildung von Niederschlagen 

 harziger Korper aus Weingeist begleiten, Poggend. Ann. xciv, pp. 447-459, 1855; 

 etc. Some at least of TomHnson's cohesion-figures arise, according to van Mens- 

 brugghe, from the disengagement of minute bubbles of gas, when a fluid holding 

 gases in solution comes in contact with a fluid of lower surface-tension. The whole 

 phenomenon is of great interest and various appearances have been referred to 

 it, in biology, geology, metallurgy and even astronomy: for the flocculent clouds 

 in the solar photosphere shew an analogous configuration. (See letters by Kerr 

 Grant, Larmor, Wager and others, in Nature, April 16 to June 11, 1914; also 

 Rayleigh, Phil. Mag. xxxii, p. 529, 1916; G. T. Walker, Clouds, natural and 

 artificial,' i?oz/a/ Inst. 8 Feb. 1935; etc.) In many instances, marked by strict 

 symmetry or regularity, it is very possible that the interference of waves or ripples 

 may play its part in the phenomenon. But in the majority of cases, it is fairly 

 certain that localised centres of action, or of diminished tension, are present, such 

 as might be provided by dust-particles in the case of Benard's experiment (cf. 

 infra, p. 503). 



t Quincke, Ueber physikalische Eigenschaften diinner fester Lamellen, Sitzungsb, 

 Berlin. Akad. 1888, p. 789; Ueber ansichtbare Eigenschaften, etc., Ann. d. Physik, 

 1920, p. 653. Quincke found that "sehr kleine Menge fremder Substanz haben 

 eine grosse Einfluss auf die Bildung der Schaumwande." 



