A manual of dental anatomy, human and comparative / by Charles S. Tomes ; edited by H.W. Marett Tims and A. Hopewell-Smith.

  • Tomes, Charles S. (Charles Sissmore), Sir, 1846-1928.
Date:
1914
Catalogue details

Licence: In copyright

Credit: A manual of dental anatomy, human and comparative / by Charles S. Tomes ; edited by H.W. Marett Tims and A. Hopewell-Smith. Source: Wellcome Collection.

Provider: This material has been provided by The University of Glasgow Library. The original may be consulted at The University of Glasgow Library.

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    would terminate in blind extremities near either end of the embryo. At the hinder end this sometimes happens, and the result is that the animal is born with an imperforate anus. At this stage the parts are named the foregut (the blind anterior end), the midgut, and the hindgut. The mouth is formed later by an invagination of the epiblast from the surface, so as to make a pouch (stomodoeum) reaching down to the foregut, but it is still some time before the septum between them breaks down. The anus is formed in a similar way, the invagination (proctodeum) being much shallower and occurring at a later date. Thus we have in what is to be the mouth, a portion of the actual exterior or future skin of the embryo carried in, and so it ceases to be a matter for surprise that we should find in the mouth skin structures, viz., dermal denticles. It is, however, pointed out by Dr. Wortman (5), on the authority of Mr. Byder, that the teeth of those fish which have teeth upon the gill arches and pharyngeal bones, lie beyond the limits of the invaginated integument, and are therefore of hypoblastic derivation. He goes on to add that, if this be true (which is not certain), the generalisation that all teeth are modified dermal spines would be incorrect. It might be, however, an example in which identical structures have been produced in a similar manner from tissues of different origin, and in all probability be attributable to the same cause, viz., repeated stimulation of a particular point, which eventually gave rise to a calcified papilla. The more probable explanation of the origin of teeth in these positions is that they are derived from lateral invaginations of the epiblast, similar in character to the .anterior stomodoeum. These invaginations meet with lateral evaginations from the walls of the foregut. The bilaminar membrane separating the cavities of these epiblastic and hypoblastic pouches ultimately ruptures, thus giving rise to the pharyngeal gill slits, which are consequently formed in a manner comparable with that of the mouth. Likewise, the origin of the teeth upon the gill arches will be the same as of those situated in the mouth.* If the growth of the dog-fish be followed, those spines of the skin which cover the jaws become developed to a greater size than those outside. A groove, without spines, appears between the jaw and the lip, and the identity and continuity of the two become to some extent masked. No one can doubt, whether from the comparison of adult forms or from a study of the development of the parts, that the teeth of the shark correspond to the teeth of other fish, and these again to those of reptiles and mammals. It may be clearly demonstrated that the teeth * The arrangement of the pharyngeal teeth and the degree to which they are developed vary greatly. Many of these have been described by Colonel Shepherd in a series of papers which have recently appeared in the Zoologist. Eds. 1-2 .