DENTINE 281 



upon the matrix. In the finished dentine of the Wombat's 

 incisor this lamination is particularly well marked and 

 conspicuous. That this lamination or stratification of 

 the dentine is due to the structure of the calcospherites is, 

 we think, very evident, and is the result of the rhythmic 

 or periodic deposit of the lime salts in the colloid. The 

 phenomena of the production of Liesegang's rings (fig. 180) 

 gives strong evidence of this, showing that the phenomena 

 of diffusion are periodic. As Leduc says (14) : ' The growth 

 of an osmotic production shows itself not as a continuous 

 process but periodically.' D'Arcy Thompson (20) also refers 



FIG. 180. Liesegang's rings. Stratification by periodic diffusion. (Pro- 

 duced by diffusion from a drop of silver nitrate in a solution of gelatine 

 to which a drop of sodium arsenite has been added. ) From Leduc. 



to this when he says : ' Among these various phenomena, 

 the concentric striation observed in the calcospherite has 

 acquired a special interest and importance. It is part of 

 a phenomenon now widely known and recognized as an 

 important factor in colloid chemistry under the name of 

 Liesegang's rings.' These rings are formed when a salt, 

 such as bichromate of potash, is placed on gelatine poured 

 upon a glass plate, the diffusion of the salt in the colloidal 

 gelatine solution showing a rhythmic deposit, leading to the 

 formation of concentric rings, and it is this rhythmic deposit 

 which occurs in the formation of the calcospherite in the 

 colloid. As Leduc says, all the phenomena of life are periodic, 1 

 although Mr. Carter states, referring to Leon Williams's 



1 For a discussion of periodicity and rhythmic deposit see Leduc 's 

 Mechanism of Life, chap, vi, p. 67, and D'Arcy W. Thompson's Growth and 

 Form, pp. 427-31. 



