The works of Francis Maitland Balfour, Volume 3 (of 4)

by Francis M. Balfour

Fig. 427. Diagram shewing the division of the primitive buccal cavity into the respiratory section above and the true mouth below. (From Gegenbaur.)
p. palatine plate of superior maxillary process; m. permanent mouth; n. posterior part of nasal passage; e. internasal septum.

An internasal septum ([fig. 427], e) may more or less completely 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 posterior 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 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 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 structures, they are nevertheless found in Teleostei and Ganoidei 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.