DEVELOPMENT OF TEETH. 507 



placed upon a joapilla-shaped dentine germ which is derived 

 entirely from the mesoblast. The subsequent formation of the 

 enamel from the enamel organ, of the dentine and cement from 

 the dentine-germ, and of the tooth-sac from the adjacent meso- 

 blastic tissue are fully described in works dealing with histology 

 and development to which we must refer the reader. 



In addition to the tooth-germs of the milk and permanent 

 dentitions, already described, other tooth-rudiments, which 

 never attain full development, are formed in many mammals.* 

 These are developed as buds from the outer, side of the tooth - 

 band in precisely the same way as are the rudiments of the 

 functional teeth. There is usually only one set of these ves- 

 tigial rudiments, the relation of which to the rudiments of the 

 functional teeth varies in different mammals. In marsupials, 

 the j)ig, and the guinea-pig, the vestiges in question are formed 

 from the tooth-band' hejore the rudiments of the milk-teeth and 

 lie on the labial side of these. In the seals, hedgehog, dog, and 

 man they arise after the rudiments of the permanent teeth and 

 lie on their lingual side. In the former case these buds are re- 

 gai'ded as vestiges of a prelacteal dentition, in the latter of a 

 fost permanent series of teeth. In those groups, such as the 

 Cetacea and Marsupialia in which the dentition is functionally 

 monophyodont or nearly so, and in which traces of two or three 

 dentitions can be made out there is some dispute as to which of 

 these dentitions the functional teeth belong to, as is shown, in the 

 subjoined table. In man there are said to be four sets of tooth- 

 rudiments, viz. of the prelacteal vestigial dentition, of the milk 

 and permanent dentitions, and of the postpermanent vestigial 

 series. In short it would appear that in Mammalia there are traces 

 of four dentitions, of which never more than two become func- 

 tional.! To this extent the dental condition of mammals may be 

 said to approximate to the polyphyodont condition of reptiles. 



If the four dentitions of iVTammalia be called premilk, milk, 



* Leche, Entwick. d. Zahnsystems tier Siiugethiere, Bibliotheca 

 Zoologica, 1895. Kiikentlial, Jena. Zeitschrift, 28, 1894, p. 76. Rose, 

 Das Zahnsystem der Wirbelthiere, Ergebnisse, d. Anatotnie n. Entwick., 

 1894. Wilson and Hill, Q.J. M.S., 39. 1897, p. 427. Adloff, Je7ia. Zeit- 

 schr., 32. 1898, p. 347. Marett Tims, Journal Anat. and Physiol., 36, 1902, 

 and 37, 1903. M. F. Woodward, P. Z. S., 1893, p. 450, and 1896, p. 557. 



t It has recently been stated that in the extinct Toxodont, Nesodon, 

 there were possibly three functional incisor dentitions (W. B. Seott, 

 British Association, Cambridge meeting, 1904). 



