DEVELOPMENT OF THE ENAMEL 137 



The interesting point is that true calcospherites are 

 formed by dialysis through membranes which are formed 

 in inorganic solutions by purely physical processes. 



Graham (10), as the result of his experiments on the 

 diffusion of liquids through membranes, divided substances 

 into crystalloids and colloids crystalloids having the power 

 when in solution of passing easily through membranes ; 

 colloids, on the other hand, passing through with difficulty, 

 or not at all. The particles of a colloid solution are held 

 together by a very feeble force ; they consist of very fine 

 particles in a state of suspension, rather than solution in 

 the solvent. The exact line of demarcation, however, 

 between crystalloids and colloids is not always very pro- 

 nounced, and according to Krause there is a steady transition 

 from the crystalloids to the colloids. 



The living body is built up of substances essentially 

 colloidal in character. These substances may exist either 

 in the solid condition, when they are called ' gels ', or in the 

 fluid state, when they are spoken of as ' sols '. 



If the colloids throughout the body are brought into the 

 condition of irreversible ' gels ', life ceases, but there are 

 conditions in which the transition of ' sols ' into ' gels ' 

 does not lead to death, but to the formation of the important 

 structures before referred to, the membranes of precipitation. 

 This partial gelation of the colloids takes place when two 

 different colloids, or a colloid and a crystalloid, come into 

 contact. 



These membranes of precipitation of Traube (27) are 

 generally considered to be permeable to water only, 

 but, as has been evidenced in the organic tissues of plants, 

 salts are capable of passing through them, and Loeb l is of 

 opinion that cell walls are not impermeable to salts, but 

 there is only a difference in the rate of diffusion, many 

 salts diffusing only very slowly into the protoplasm. 

 Leduc (15) says : ' Osmotic membranes were formerly 

 called semi-permeable membranes, being regarded as 

 membranes which allow water to pass through them but 



not be a proof of the absence of lime, as calcium salts are present in ordinary 

 specimens of sodium silicate. 



1 Dynamics of Living Matter, 1906. 



