506 THE FORMS OF TISSUES [ch. 



We learn from these experiments of Benard and others how 

 similar distributions of force, and identical figures of equihbrium, 

 may arise through different physical agencies. We see that patterns 

 closely analogous to those of. living cells and tissues may be due 

 to very different causes ; and we may be led to scrutinise anew, with 

 an open mind, various histological configurations whose origin is 

 doubtful or obscure. The chitinous shells of certain water-fleas 

 (Cladocera) are beset with a roughly hexagonal pattern, and each 

 little chitinous polygon is supposed to correspond to, and to be 

 formed by, an underlying "hypodermis" cell*. But we presently 

 discover that the existence of these hypodermis-cells is merely 

 deduced from the polygons themselves and from a coincident 



Fig. 185. Soap-froth under pres.sure. After Rhumbler. 



distriburion of pigment; it might not be amiss to look again into 

 the development of the pattern, with an open mind as to the 

 possibihty of its being a purely physical phenomenon. Nor need 

 we by any means assume that the calcareous prisms of a molluscan 

 shell are necessarily derived from, or associated with, a hke number 

 of histological elements. 



In a soap-froth imprisoned between two glass plates w^e have a 

 symmetrical system of cells which appear in optical section (Fig. 

 185, B) as regular hexagons; but if we press the plates a little 

 closer together the hexagons become deformed and flattened. The 



* Cf. F. Claus, Zur Kenntniss. . .des feineren Baues der Daphniden, Ztschr. f. 

 loiss. Zool. XXIII, XXVII, pp. 3(i2-402, 1876; Ernest Warren, Relationship between 

 size of cell and size of body in Dapknia magna, Biometrika, (3) ii, pp. 255-259, 

 1902; Fritz Werner. Die V'eranderung der Schalenform und der Zellenaufbau bei 

 Scaphohberis. Int. Berne der yes. Hydrobioloyie, pp. 1-20, 1923. 



