1882.] the Enamel of the Teeth of Vertebrates. 



157 



The existence of a basement membrane or menibrana prceformativa, 

 said by some to cover the tooth papilla, is generally considered 

 doubtful. It is asserted, 1st, that the newly-formed layer of enamel 

 has been mistaken for it (Waldeyer) ; 2, that the first-formed layer of 

 dentine has been mistaken for it (Lowve) ; 3, that the " appearances 

 described are capable of a different interpretation " (Tomes). 



The present investigation was undertaken at the suggestion of 

 Professor Huxley with the hope of determining — or at least of getting 

 more light upon — firstly, the history of the various membranes — the 

 cuticula dentis, the so-called newly-formed layer of enamel, the mem- 

 brana prsef ormativa, and the first-formed layer of dentine ; secondly, 

 the origin of the enamel. 



The present paper gives the results which have been arrived at 

 concerning these points. 



The nature of dentine will form the subject of a second paper. 



Since it is well established that the teeth and placoids of the 

 Plagiostomi are homologous with the teeth of the higher vertebrates, 

 illustrations from these will be used as well as from mammalian teeth, 

 adult and embryonic. 



The drawings described have been made by the aid of the camera. 



1. The Cuticula Dentis. — A membrane covering the enamel of 

 young teeth has been repeatedly described as homogeneous and reticu- 

 lated, or with " zellzeichnungen" supposed to have been made by 

 adjoining cells. Huxley* says of this membrane: "It is perfectly 

 clear and transparent, and under a high power exhibits innumerable 

 little ridges upon its outer surface, which bound spaces sometimes 

 oval, sometimes quadrangular, and about 1 5 X 00 of an inch in diameter. 

 In the frog its surface is in parts reticulated as in man ; in the 

 mackerel and skate I have been unable to find any such reticulation. 

 In the calf a similar membrane may be demonstrated, but it is much 

 more delicate, and I have not seen the peculiar areolae on its surface." 



This account agrees with the description given by other writers of 

 ^. membrane found lying upon the enamel, as also with my own 

 observations. 



The membrane, then, may be reticulated, entirely, in parts only, or 

 not at all. It varies in thickness in the same animal, and as well as 

 in different animals, and may be thick or delicate. It has been found 

 by myself, as well as by other observers, upon the young tooth, both 

 before and after its eruption. Hitherto it has not been figured, f 

 neither has its history been determined ; hence the various opinions 

 as to its nature and origin. Waldeyer]: declared it to be the newly - 



* Huxley, " Quarterly Journal of Microscopical Science," 1853, pp. 157, 158. 

 f A drawing, by Nasmyth, lias since been found, of the inner surface of this 

 membrane, showing the impressions of the enamel prisms. 

 X Waldeyer, " Handbuch," 1869. 



