430 APPENDIX. 



It must be understood, however, that the latter two structures 

 are continuous, and that when the dentine is said to be depo- 

 sited 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 structureless, which may, however, arise from the small 

 quantity of calcareous matter which it contains : subsequently, 

 minute cavities, irregular in form, and ^m tn °f an i ncn apart, 

 appear in it; and these corresponding 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 tegu- 

 mentary organs of many of the lower animals (Fishes, Articu- 

 lata, Mollusca) are formed, has afforded the fullest confirmation 

 of this theory of the development 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 

 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 — that in a Molluscan 

 shell, for instance (e. g. Trigonia), we may have a superficial 

 membranous layer corresponding with Nasmyth's membrane, 

 a deeper prismatic layer, whose prisms precisely resemble 

 those of enamel on a large scale, and an internal laminated 

 tubular layer, corresponding with dentine; and knowing, 

 further, that these varieties of structure thus arranged are 

 (whatever view we take of shell-structure) nothing but the 

 result of the different modes in which calcific deposit has 

 successively taken place in the same organ, it is sufficiently 

 obvious that there are abundant analogical grounds for con- 

 sidering the enamel and the dentine as modifications of one 

 and the same tissue. 



Nor does the structure of the enamel in the Fish or the 



