324 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



effective. In some of the lower fishes (many Elasmobranchs, e.g. 

 Acanthias) teeth of a simple character, practically unmodified 

 placoid elements, are still to be found scattered over the roof of the 

 buccal cavity and even extending back into the pharynx. Un- 

 fortunately the development of these has not been worked out in 

 detail. In Teleostean fishes however a very simple type of tooth 

 development has been described e.g. in the Pike (Esox lucius).^ The 

 teeth are here no longer scattered equally over the buccal lining ; 

 they are restricted to the dentary, maxilla, vomer, palatine, and 

 the inner surface of the visceral arches. The teeth on the roof of 

 the mouth arise as simple conical dermal papillae which project into 

 the epidermis and develop enamel, dentine, and an irregular trabe- 



A. 



B. 



If *i^m« 





Fig. 161. — Early stage in the development of the tootli in (A) C&ratodus 

 and (B) Lepidosiren. (A after Semon, 1899.) 



d, dentine ; od, odontoblasts. 



cular bony base on the same general lines as described above for the 

 typical placoid element. 



Kelatively primitive conditions are found again in the Dipnoi 

 and Amphibia (Urodela and Gymnophiona) in the latter of which 

 the teeth may be very widely distributed e.g. on premaxilla, maxilla, 

 vomer, palatine, pterygoid, parasphenoid (Spelerpes), as well as on' 

 the dentary and occasionally on the splenial. In the simplest 

 cases the tooth originates as a simple conical or rounded dermal 

 papilla which projects upwards into the ectoderm (cf. Ceratodus, 

 Fig. 161, A) but in other cases an onward step has been made and 

 the " ectoderm " in the region where the papilla develops tends 

 to grow down below the general < level of the ectoderm into the 

 underlying connective tissue of the dermis (cf. Lepidosiren, Fig. 

 161, B). 



In the Amniota this tendency becomes more pronounced, the 

 ectoderm covering the tooth-germ not merely projecting downwards 

 into the underlying mesenchyme but hecoming constricted off from 



