V] OF RETICULATE OR WRINKLED CELLS 417 



Many species of Lagena are complicated and beautified by a 

 pattern, and some by the superaddition to the shell of plane 

 extensions or "wings." These latter give a secondary, bilateral 

 symmetry to the little shell, and are strongly suggestive of a phase 

 or period of growth in which it lay horizontally on the surface, 

 instead of hanging vertically from the surface-film : in which, that 

 is to say, it was a floating and not a hanging drop. The pattern 

 is of two kinds. Sometimes it consists of a sort of fine reticulation, 

 with rounded or more or less hexagonal interspaces: in other cases 

 it is produced by a symmetrical series of ridges or folds, usually 

 longitudinal, on the body of the flask-shaped cell, but occasionally 

 transversely arranged upon the narrow neck. The reticulated and 

 folded patterns we may consider separately. The netted pattern 

 is very similar to the wrinkled surface of a dried pea, or to the more 

 regular wrinkled patterns on poppy and other seeds and even pollen- 

 grains. If a spherical body after developing a "skin" begin to 

 shrink a little, and if the skin have so far lost its elasticity as to 

 be unable to keep pace with the shrinkage of the inner mass, it will 

 tend to fold or wrinkle; and if the shrinkage be uniform, and the 

 elasticity and flexibihty of the skin be also uniform, then the amount 

 of foldings will be uniformly distributed over the surface. Little 

 elevations and depressions will appear, regularly interspaced, and 

 separated by concave or convex folds. These being of equal size 

 (unless the system be otherwise perturbed), each one wiU tend to 

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

 limit, the intermediate boundary-walls, or folds, will be found 

 converted into a more or less regular pattern of hexagons. To these 

 symmetrical wrinkles or shrinkage-patterns we shall return again. 



But the analogy of the mechanical wrinkHng of the coat of a 

 seed is but a rough and distant one; for we are deahng with 

 molecular rather than with mechanical forces. In one of Darling's 

 experiments, a little heavy tar-oil is dropped on to a saucer of 

 water, over which it spreads in a thin film shewing 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 aU probability at work in a surface-film 



