790 APPENDIX. 



tooth ; or with the surface of the cement, in a fully developed one. 

 Nasmyth's membrane, in fact, is at first the membrana preformativa 

 more or less altered. 



We hold this statement to be incontrovertible, nor less so the corre- 

 lated doctrine, that the enamel and cement, as well as the dentine, are 

 developed beneath Nasmyth's membrane; between it and the pulp ; that 

 the enamel is, consequently, during the whole course of its formation, 

 separated from the enamel-organ by Nasmyth's membrane ; and that, 

 therefore, the direct conversion of the long cylinders of the epithelium 

 into the fibres of the enamel, strongly as their mutual resemblance may 

 suggest the notion, is. to say the least, highly improbable. 



Thirdly, we have met with no facts in opposition to what we have 

 stated with regard to the mode of development of dentine and the re- 

 lation of the latter to the pre-existing elements of the pulp, and we 

 believe that there is every reason to regard what we have there called 

 the " Deposition theory," as an established position. According to this 

 theory, the dentine is not the result of the conversion of the elements of 

 the pulp ; the endoplasts of the latter never becoming engaged in the 

 calcareous deposit, as they are in bone ; but the young dentine is formed 

 by a deposit of transparent calcareous granules in a thin layer between 

 the pulp and the membrana preformativa. It must be understood, how- 

 ever, that the latter two structures are continuous, and that when the den- 

 tine is said to be deposited between them it is not meant that any real 

 interval exists, but only that the outer portion of the periplast of the 

 pulp, of which the membrana preformativa constitutes a part, increases 

 and receives a calcareous deposit without any corresponding implication 

 of the endoplasts of the pulp. 



The thinnest and youngest layer of the dentine appears to be struc- 

 tureless, which may, however, arise from the small quantity of calcareous 

 matter which it contains : subsequently, minute cavities, irregular in 

 form, and l-5000th of an inch apart, appear in it ; and these corre- 

 sponding with one another in successive layers of the dentine, become 

 the dentinal tubuli. The appearance of walls, &c., to these tubuli, we 

 consider to be the result of a subsequent differentiation in the dentine. 



A careful study of the mode in which the dentine-like tegumentary 

 organs of many of the lower animals (Fishes, Articulata, Mollusca) are 

 formed, has afforded the fullest confirmation of this theory of the deve- 

 lopment of dentine, and we would recommend those who have any doubt 

 upon the subject to study the development of the spines of the Skate, 

 or that of the shell of the Crab or Lobster. 



The mode of development of the enamel appears to us to be a very 

 difficult subject, and requires to be most carefully studied. Taking 

 into consideration the facts that a distinction of a superficial and a deep 

 layer of calcified tissue is very general in the tegumentary organs 



