ON THE DEVELOPMENT OP TEFTH. 45 



An observation of my own ujoon the tooth-germs of the 

 armadilloj and one of Mr, Turner's upon that of a foetal 

 narwalj had served to indicate the probability that within the 

 limits of the mammalia class the presence of an enamel organ 

 would be found universal, as in neither of these animals is 

 there any enamel upon the completed tooth. 



My own further researches upon the tooth development of 

 fish, batrachia, and reptiles has shown that the presence of 

 an enamel organ is quite universal, and that the ingrowth of 

 epithelial cells (enamel germ of KoUiker) is the earliest 

 recognisable step towards the formation of any and every 

 tooth-germ. 



A common English lizard may be taken as our example 

 from the class of reptiles. I have never had an opportunity 

 of observing the development of the first-formed tooth-germs, 

 but the manner in Avhich the successional teeth are formed 

 may be very completely worked out. 



In man the tooth-germs (even of the permanent teeth) are 

 formed during the foetal period, and they are at first very 

 close to the epithelial surface. But in reptiles, where 

 the development of new teeth is going on throughout the 

 creature's life, the new germs are formed at a considerable 

 distance from the surface. Hence the inflection of epithe- 

 lium penetrates to a far greater depth before it reaches the 

 spot where the dentine papilla is to be formed (see c in fig. 3), 

 so that it plays an even more conspicuous part than in a mam- 

 malian jaw. When it has reached through the whole thick- 

 ness of the soft parts, nearly to the bone, a set of changes 

 closely similar to those previously described takes place ; 

 fig. 2 represents a young tooth-germ of a lizard. In its 

 further development some points of difference which would 

 serve to distinguish it from mammalian tooth-sacs arise. In 

 the first place a less definite capsule is formed; the surround- 

 ing connective tissue becomes pushed on one side and so forms 

 a slight adventitious capsule, but this is all. 



The enamel organ also in its further development becomes 

 differentiated into an internal epithelium (enamel cells) and 

 an external e^iitheliimi, but these are at no period separated 

 from one another by any loose network of stellate cells (the 

 reticulum). The dentine papilla presents no marked diver- 

 gence from the characters met with in those of mammalia. 



The tooth when completed moves up into position, tooth- 

 sac and all ; but it becomes fixed to the bone by a rapid de- 

 velopment of coarse bone which takes place outside the 

 limits of the sac, and not by a calcification of a part of its 

 own capsule, as has been generally supposed. 



