V] OF RETICULATE OR WRINKLED CELLS 259 



folding will be uniformly distributed over the surface. Little 

 concave depressions will appear, regularly interspaced, and 

 separated by convex folds. The little concavities being of equal 

 size (unless the system be otherwise perturbed) each one will tend 

 to be surrounded by six others ; and when the process has reached 

 its hmit, the intermediate boundary-walls, or raised folds, will be 

 found converted into a regular pattern of hexagons. 



But the analogy of the mechanical wrinkling of the coat of 

 a seed is but a rough and distant one ; for we are evidently dealing 

 with molecular rather than with mechanical forces. In one of 

 Darlmg's experiments, a little heavy tar-oil is dropped onto a 

 saucer of water, over which it spreads in a thin film showing 

 beautiful interference colours after the fashion of those of a soap- 

 bubble. Presently tiny holes appear in the film, which gradually 

 increase in size till they form a cellular pattern or honeycomb, 

 the oil gathering together in the meshes or walls of the cellular 

 net. Some action of this sort is in all probability at work in a 

 surface-film, of protoplasm covering the shell. As a physical 

 phenomenon the actions involved are by no means fully under- 

 stood, but surface-tension, diffusion and cohesion doubtless play 

 their respective part? 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 



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

 described by Tomlinson in various surface-films (Phil. Mag. 1861 to 1870) ; to 

 the "tesselated structure" in hquids described by Professor James Thomson in 

 1882 [Collected Papers, p. 136); and to the tourbiUo7is cellulaves of Prof. H. Benarcl 

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

 Rev. geae'r. des Sci. xi, p. 1268, 1900; cf. also E. H. Weber. Prggend. Ann. 

 xciv, p. 452, 1855, etc.). The 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 Cirant, Larmor, Wager and others, in Nature, 

 April 16 to June 11, 1914.) 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 

 locahsed centres of action, or of diminished tension, are present, such as might be 

 provided by dust-particles in the case of Darling's experiment (cf. infra, p. 590). 



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