GROWTH, METAMORPHOSIS AND DEVELOPMENT 259 



In the first place they resemble most organic fibers by being 

 doubly refractive (L. JOST*) even though this double refraction is 

 caused by different conditions than in organized structures. In 

 contradistinction to natural fibers, the substance is isotropic; it is 

 only its lamellated structure which produces that type of double re- 

 fraction (H. AMBRONN*) which is found in the siliceous envelopes of 



FIG. 45. Piece of doubly diffracting fibrous alumina. Magnified X 440. 

 (After H. Wislicenus.) 



diatoms (e.g., pleurosigma and amphipleura) and probably also in 

 tabasheer, the colloidal silicic acid which occurs in the internodes of 

 some species of bamboo. 



The Genesis of Structures. 



What phase is for the physical chemist, cells and tissues are for 

 the biologist. Like phases, cells and tissues are "portions of a 

 structure separated from one another by physical interfaces" (WiL- 

 HELM OSTWALD'S definition of phase). The interface may consist 

 of an invisible transition layer (see p. 280). The interface is most 

 evident when it consists of a visible membrane. Such a membrane 

 whether visible or invisible is always a structure poor in water. At 



