THE TEETH. 



two divisions thus formed open into a common cavity behind. 

 The horizontal septum, on the development within it of an 

 osseous plate, constitutes the hard palate. 



An internasal septum (fig. 427, e) may more or less com- 

 pletely divide the dorsal cavity into two canals, continuous 

 respectively with the two nasal cavities. 



In Mammalia a posterior prolongation of the palate, in which 

 an osseous plate is not formed, constitutes the soft palate. 



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, 

 becoming, on the atrophy of these 

 structures, converted into the pos- 

 terior part of the buccal cavity. 



The organs derived from the 

 buccal cavity are the tongue, the 

 various salivary glands, and the 



teeth ; but the latter alone will en- 



.. .. i FIG. 427. DIAGRAM SHEW- 



gage our attention here. ING THE ^VISION OF THE PR i M - 



The teeth. The teeth are to be ITIVE BUCCAL CAVITY INTO THE 



. . RESPIRATORY SECTION ABOVE 



regarded as a special product of the AND THE TRUE MOUTH BELOW. 



oral mucous membrane. It has been ( From 

 shewn by Gegenbaur and Hertwig J 



that in their mode of development mouth ; n. posterior part of nasal 

 ,1 ,. .1 i i ^1 i i passage; e. internasal septum. 



they essentially resemble the placoid 



scales of Elasmobranchii, and that the latter structures extend 



in Elasmobranchii for 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 ingrowth 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 Ganoidei on the hyoid 



