330 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



succession of tooth-germs are produced from this strand, each one 

 lying to the inner side of a functional tooth. As the successional 

 tooth develops it causes absorption of the inner wall of the functional 

 tooth, and gradually comes to lie within the base of the latter. 

 Finally the old tooth is shed and its successor remains in its, 

 place. 



In toothed Birds, so far as is known (Hesperomis, Marsh, 1880), 

 the tooth replacement seems to have taken place in the same way 

 as in Crocodiles. In modern birds a slight transitory ectodermal 

 thickening has been interpreted as the vestige of a dental lamina 

 (e.g. Terns— Rose, 1892; Carlsson, 1896) but the evidence is not 

 convincing. Careful researches in this direction in the less highly 

 specialized birds are highly desirable. 



. Tooth-Plates. — In many of the lower Vertebrates instead of, 

 or in addition to, 'teeth of the ordinary conical shape-adapted for 

 piercing, there are present massive plate-like structures adapted for 

 crushing. Large tooth-plates of this kind were conspicuous structures 

 in many of the extinct fishes and lower Tetrapoda. Amongst living 

 Vertebrates they are exemplified by many of the Skates and Rays, by 

 the Holocephali and by the Dipnoi. Embryological study has shown 

 that these plate-like teeth may arise in either of two possible ways. 



In the Skates and in the Holocephali the tooth-plate is a single 

 much enlarged and flattened tooth. In Gallorhynchus Schauinsland 

 (1903) describes how the tooth-plate originates in a widespreading 

 dental papilla of a depressed dome shape. The outer layer of this 

 develops a cap of dentine in the ordinary way. Below and con- 

 tinuous with this there develops a trabecular spongework of calcified 

 tissue which shows a transition from ordinary dentine in its super- 

 ficial parts to a tissue closely resembling normal bone except that the 

 cell-bodies remain superficial, only their branching processes becoming 

 embedded in the calcified material. As development goes on the 

 individual trabeculae become thicker and more numerous and the 

 intervening meshes filled with ordinary vascular mesenchyme 

 become less and less conspicuous until the tooth as a whole assumes 

 its definitive strong and massive character. No typical enamel 

 seems to be formed but Schauinsland points out that the enamel 

 epithelium seems to exercise a distinct modifying influence over the 

 superficial layer of dentine which becomes hard and glassy (Vitro- 

 dentine) wherever it is in contact with the enamel epithelium. 



In the case of Lung-fishes Semon (1899) has given a beautiful 

 demonstration, which I can fully confirm, that the tooth-plates 

 originate not by the enlargement and modification of single teeth 

 but by the fusion of a number of originally separate denticles. The 

 evidence of Palaeontology it may be mentioned is in complete agree- 

 ment with the embryological evidence furnished by Oeratodus on 

 this point and we may take the latter as a particularly good example 

 of the recapitulation of phylogenetic evolution during the develop- 

 ment of the individual. 



