VII] OF HEXAGONAL SYMMETRY 507 



change is from a more to a less probable configuration — the 

 entropy is diminished; and if we apply no further pressure the 

 tension of the films adjusts itself again, and the system recovers 

 its former symmetry*. 



The epithelial hning of the blood-vessels shews a curious and 

 beautiful pattern. The cells seem diamond-shaped, but looking 

 closer we see that each is in contact (usually) with six others; they 

 are not rhombs, or diamonds, but elongated hexagons, pulled out 

 long by the growth of the vessel and the elastic traction of its walls. 

 The sides of each cell are curiously waved, and a simple experiment 

 explains this phenomenon. If we make a froth of white-of-egg 

 upon a stretched sheet of rubber, the cells of the froth will tend to 



Fig. 186. Sinuous outlines of epithelial cells, a, endothelium of a blood-vessel; 

 h, epidermis of hnpatiens; r, epidermal cells of a grass (Festuca). 



assume their normal hexagonal pattern ; but relax the elastic mem- 

 brane, and the cell-walls are thrown into beautiful sinuous or wavy 

 folds. The froth-cells cannot contract as the rubber does which 

 carries them, nor can the epithelial cells contract as does the 

 muscular coat of the blood-vessel ; in both cases alike the cell- walls 

 are obliged to fold or wrinkle up, from lack of power to shorten. 

 The epithelial cells on the gills of a mussel I are wrinkled after 

 the same fashion; but the more coarsely sinuous outlines of the 

 epithelium in many plants is another story, and not so easily 

 accounted for. 



The hexagonal pattern is illustrated among organisms in count- 

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



* That everything is passing all the while towards a ''more probable state'' is 

 known as the "principle of Carnot," and is the most general of all physical laws 

 or aphorisms. 



t Cf. James Gray's Experimental Cytology, p. 252. 



