G38 



THE TKFAII. 



[ir-~\]J 



Fig. 427. Diagbam shewing 

 the division of the primitive 

 buccal cavity into the kespi- 

 ratoby section above and the 

 TRUE MOUTH BELOW. (From Ge- 

 genbaur. ) 



p. palatine plate of superior 

 maxillary process ; m. permanent 

 mouth ; v. posterior part of nasal 

 passage; e. internasal septum. 



The second change in the Amniota, which also takes place in 

 some Amphibia, is caused by the section of the mesenteron into 



which the branchial pouches open, be- 

 coming, on the atrophy of these struc- 

 tures, converted into the posterior part 

 of the buccal cavity. 



The organs derived from the buccal 

 cavity are the tongue, the various sali- 

 vary glands, and the teeth ; but the lat- 

 ter alone will engage our attention here. 

 The teeth. The teeth are to be 

 regarded as a special product of the 

 oral mucous membrane. It has been 

 shewn by Gegenbaur and Hertwig that 

 in their mode of development they es- 

 sentially resemble the placoid scales of 

 Elasmobranchii, and that the latter 

 structures extend in Elasmobranchii 

 f )r a certain distance into the cavity of 

 the mouth. 



As pointed out by Gegenbaur, the teeth are therefore to be 

 regarded as more or less specialised placoid scales, whose presence in 

 the mouth is to be explained by the fact that the latter structure is 

 lined by an invagination of the epidermis. The most important 

 developmental point of difference between teeth and placoid scales 

 consists in the fact, that in the case of the former there is a special 

 ino-rowth of epiblast to meet a connective tissue papilla which is not 

 found in the latter. 



Although the teeth are to be regarded as primitively epiblastic struc- 

 tures, they are nevertheless found in Teleostei and Gnnoidei on the hyoid 

 and branchial arches ; and very possibly the teeth on some other parts of 

 the mouth are developed in a true hypoblastic region. 



The teeth ai-e formed from two distinct organs, viz. an epithelial cap 

 and a connective tissue papilla. 



The general mode of development, as has been more especially shewn 

 by the extended r.'searches of Tomes, is practically the same for all Verte- 

 br.ita, and it will be convenient to describe it as it takes place in Mam- 

 malia. 



Along the line where the teeth are about to develop, there is formed 

 an epithelial ridge projecting into the subjacent connective tissue, and 

 derived from the innermost columnar layer of the oral epithelium. At 

 the points where a tooth is about to be formed this ridge undergoes special 

 changes. It becomes in the first place somewhat thickened by the deveh)p- 

 ment of a number of rounded cells in its interior; so that it becomes 

 constituted of (1) an external layer of columnar cells, and (2) a central 

 core of rounded cells; both of an epithelial nature. In the second place 

 the organ gradually assumes a dome-shaped form (ti!,'. 428, e), and covers 

 over a pajnlla of the subepithelial connective tissue (;;) which has in the 

 meantime been devolo[)e(.]. 



