DEVELOPMENT OF THE ENAMEL 143 



Pinna and in other shells the membrane left after decalcifica- 

 tion represented the organic cells in which the deposit of 

 the lime salts had taken place. The view now generally 

 held with regard to shell formation is that it is due to 

 a process of secretion. This is more in accordance with 

 modern views regarding the calcification of the teeth, 

 which is considered to be produced by deposit in a material 

 separated and secreted by the cells, and not by the direct 



FIG. 82. Carapace of Crab showing striation and incorporation of 

 spherites. Dark ground illumination. (x500.) 



conversion of the cell into the calcified substance by inter- 

 stitial deposit. The organic remainder after decalcification 

 of a shell of Pinna would not, then suggest that these 

 hexagonal outlines indicate cells, but that they were due to 

 the remains of the fibrillar organic material present in all 

 shells, in which the deposit takes place the remains of the 

 basis substance between the crystalline calcareous plates by 

 which the prisms of this shell are built up. 



In the brachiopod Terebratula, the fibrillar substance 

 is very clearly to be seen in, the mantle, and also in 

 the remains of the decalcified shell there is no evidence of 



