The Microscope. 39 



developed in much the same way, only the cord is longer and 

 more slender. 



It arises from the same infant layer, generally from the same 

 cord as does the temporary tooth, and directs its way down be- 

 tween the tooth and follicular wall, behind the temporary tooth ; 

 there it undergoes the same process as its antecedent. (See Fig. 

 6 c). 



The deposition of enamel and dentine is from one end only 

 of their respective cells, and only in a direction toward each 

 other. In this respect bone development differs very materially, 

 for we have noticed that in the development of bone ihe osteo- 

 blasts do not become themselves calcified, but the lime salts are 

 deposited around the osteoblasts in the calcospherules, thus 

 completely encasing them. In the development of dentine, 

 although the odontoblasts do not themselves become calcified, 

 yet the dentine is deposited only from around their fibril ends 

 in a tubular form. The deposition of enamel is in much the 

 same way ; the ameloblasts are not themselves calcified, but they 

 deposit the prismatic enamel from the end next to the odoto- 

 blasts. (See Fig. 8, e, d). 



The dentine consists of a solid mass of calcified tissue ar- 

 ranged in a tubular form around the fine processes of the odonto- 

 blasts. The enamel is a still more condensed structure of cal- 

 carous matter consisting of hexagonal prisms arranged side by 

 by side and very closely together. " I look upon enamel as 

 nothing more or less than a coat of mail supplied by Nature to 

 protect the dentine and subserve the process of mastication " 

 (Sudduth). 



Before the tooth issues from the bed of the jaw, the outer 

 tunic has completely disappeared, and the enamel upon the 

 apex of the tooth having been fully formed, the ameloblasts 

 which produced it, having served their purpose, have also dis- 

 appeared, so that whatever injury is done later to the en- 

 amel no provision is made for its repair. Then briefly to 

 recapitulate. We have noticed how that bone originated from 

 one class of cells, the enamel from another; how that early 

 in embryonic life a difference is seen in the appearance 

 and course taken by each class of these cells ; how that work- 

 ing together in one organ they produce results unlike the 

 product of either separately. And upon the theory that one 



