DEVELOPMENT. 185 



dense, and minutely fibrous in all the reptiles which have their teeth 

 defended by this substance. 



74. Development. — The teeth of reptiles are never completed, as in 

 certain fishes, at the first or papillary stage ; but the pulp sinks into 

 a follicle, and becomes inclosed by a capsule : the process of develop- 

 ment, however, never oflfers the eruptive stage, in the sense in which 

 this is usually understood, as signifying the extrication of the young 

 tooth from a closed alveolus. 



The completion of a tooth is soon followed by preparation for 

 its removal and succession : the faculty of developing new tooth-germs 

 seems to be unlimited in the present class, and the phenomena of 

 dental decadence and replacement are manifested at every period 

 of life : the number of teeth is generally the same in each successive 

 series, and the difference of size presented by the teeth of different 

 and distant series is considerable. 



The new germ is always developed, in the first instance, at the 

 side of the base of the old tooth, never in the cavity of the base ; the 

 crocodiles form no exception to this rule. The poison-fangs of ser- 

 pents succeed each other from behind forwards ; in almost every other 

 instance, the germ of the successional tooth is developed at the inner 

 side of the base of its 'predecessor. In the frog, the dental germ 

 makes its appearance in the form of a papilla developed from the 

 bottom and towards the outer side of a small fissure in the mucous 

 membrane or gum that fills up the shallow groove at the inner side 

 of the alveolar parapet and its adherent teeth : the papilla is soon 

 enveloped by a capsular process of the surrounding membrane : 

 there is a small enamel pulp developed from the capsule opposite the 

 apex of the tooth ; the deposition of the earthy salts in this mould is 

 accompanied by ossification of the capsule, which afterwards proceeds 

 pari passu with the calcification of the dentinal papilla or pulp : 

 so that, with the exception of its base, the surface of the uncalcified 

 part of the pulp alone remains normally unadherent to the capsule. 



As the tooth acquires hardness and size it presses against the 

 base of the contiguous attached tooth, causes a progressive absorption 

 of that part, and finally undermines, displaces and replaces its pre- 

 decessor. The number of nascent matrices of the successional teeth 

 is so great in the frog, and they are crowded so close together, that it 



