VII] OF HEXAGONAL SYMMETRY 321 



cease. After equilibrium is attained, and when the gelatinous 

 mass is permitted to dry, we have an artificial tissue of more or 

 less regularly hexagonal "cells," which simulate in the closest way 

 an organic parenchyma. And by varying the experiment, in ways 

 which Leduc describes, we may simulate various forms of tissue, 

 and produce cells with thick walls or with thin, cells in close 

 contact or with wide intercellular spaces, cells with plane or with 

 curved partitions, and so forth. 



The hexagonal pattern is illustrated among organisms in count- 

 less cases, but those in which the pattern is perfectly regular, by 

 reason of perfect uniformity of force and perfect equality of the 

 individual cells, are not so numerous. The hexagonal epithelium- 

 cells of the pigment layer of. the eye, external to the retina, are 

 a good example. Here we have a single layer of uniform cells, 

 reposing on the one hand upon a basement membrane, supported 



Fig. 124. Epidermis of Girardia. (After Goebel.) 



behind by the solid wall of the sclerotic, and exposed on the other 

 hand to the uniform fluid pressure of the vitreous humour. The 

 conditions all point, and lead, to a perfectly symmetrical result : 

 that is to say, the cells, uniform in size, are flattened out to a 

 uniform thickness by the fluid pressure acting radially ; and their 

 reaction on each other converts the flattened discs into regular 

 hexagons. In an ordinary columnar epithelium, such as that of 

 the intestine, we see again that the columnar cells have been 

 compressed into hexagonal prisms ; but here as a rule the cells 

 are less uniform in size, small cells are apt to be intercalated 

 among the larger, and the perfect symmetry is accordingly lost. 

 The same is true of ordinary vegetable parenchyma ; the originally 

 spherical cells are approximately equal in size, but only approxi- 

 mately ; and there are accordingly all degrees in the regularity and 

 symmetry of the resulting tissue. But obviously, wherever we 

 T. G. - 21 



