144 MICROSCOPIC ANATOMY OF THE TEETH 



any deposit in cells but in a very strongly defined irregular 

 network of clear homogeneous material 1 (166). 



Calcareous deposits are not then considered to be due to 

 the plastic agency of the living cell but to physical processes, 

 although, as stated by Professor D'Arcy Thompson, ' the 

 developing concretion is somehow or other so associated 

 with living cells that we are apt to take it for granted that 

 it owes its peculiarities of form to the constructive or plastic 

 agency of these ', and further, ' the appearance of direct 

 association with living cells, however, is apt to be fallacious, 

 for the actual precipitation takes place, as a rule, not in 

 actively living, but in dead, or at least inactive, tissue, that 

 is to say, in the " formed material " or matrix which (as 

 for instance in cartilage) accumulates round the living cells, 

 in the interspaces between these latter, or at least, as often 

 happens, in connexion with the cell wall or cell membrane, 

 rather than with the substance of the protoplasm itself ' (25). 



3. Calcification of the Enamel 



Two essentially different views have been held as to the 

 part taken by the cells of the enamel organ in the process 

 of calcification. By some, especially the earlier authorities, 

 it was held that enamel is produced by the actual con- 

 version of the cells into the calcified substance, and the 

 incorporation of these cells in the formed enamel. 



By others it is considered that the process is one of 

 secretion by the ameloblasts, being comparable to the 

 process of shell formation in the Mollusca, where the hard 

 tissue is produced by a secretion from the mantle of the 

 living animal. 



According to the first view the calcifying salts would be 

 deposited within the substance of the cell, which would thus 

 be converted into the calcified enamel, but recent views on 

 the subject would appear to suggest that the process is to 

 some extent a combination of the two methods ; that while 



1 Dr. Carpenter in his work on the microscope (46), published subse- 

 quently to his original paper, is disposed to agree with Professor Huxley 

 * in the belief that the entire thickness of the shell is formed as an excretion 

 from the surface of the epidermis ' . 



