288 DEVELOTMENT OF ELETIIANT's GRINDERS. 



wise explained according to the "excretion tlieory" of 

 dental development. To the obvious objection that the 

 same part is made, in this explanation, to secrete two 

 different products, Cnvier replies, that it undergoes a 

 change of tissue: "Whilst it yielded enamel only, it Avas 

 thin and transparent ; to give cement, it becomes thick, 

 spongy, and of a reddish color." The external characters 

 of the enaijiel-organ and cement-forming capsule are cor- 

 rectly defined ; only, the one, instead of being converted 

 into the other, is in fact changed into its supposed transu- 

 dation ; the enamel fibres being formed, and properly dis- 

 posed in the direction in which their chief strength is to 

 lie, by the assimilative properties of the prearranged 

 elongated, prismatic, non-nucleated cells, which take from 

 the surrounding phisma the required salts, and compact 

 them in their interior. 



Whilst this process is on foot, and before the enamel 

 fibres are firm in their position, the capsule begins to 

 undergo that change which results in the formation of the 

 thick cement ; the calcifying process commences from 

 several points, and proceeds centrifugally, radiating there- 

 from, and differing from the ossification of bone chiefly 

 in the number of these centres, which, though close to the 

 new-formed enamel, are in the substance of the inner 

 vascular surface of the capsular folds. The cells arrange 

 themselves in concentric layers around the vessels, and 

 act like those of the enamel pulp in receiving into their 

 interior the bone-salts in a clear and compact state. Dur- 

 ing this process they become confluent with each other, 

 their primitive distinctness being indicated only by their 

 persistent granular nuclei, which now form the radiated 

 Purkingian capsules. The interspaces of the concentric 

 series of confluent cells become filled with the calcareous 



