30 MICROSCOPIC ANATOMY OF THE TEETH 



At birth the sacs of the first permanent molars and those 

 of all the temporary teeth are fully formed (fig. 5), the 

 crowns of the temporary incisors are calcified, and the tip 

 of the canine and the separately formed cusps of the two 

 temporary molars are united. One cusp of the first per- 

 manent molar is calcified and seen as a minute triangle 

 (figs. 6 and 7). 



Tooth Development in Reptiles and Fish 



In Reptiles the new tooth germs are, without doubt, 

 developed from the tooth-band as in Mammalia, the succes- 

 sional teeth being given off from the continued ingrowth 

 of this band. In the osseous fish, where it is difficult to 

 trace the connexion of the germs with the tooth-band, it has 

 been considered that these germs may arise independently, 

 and Heincke has stated that new enamel organs may be 

 derived from older ones ; but this hardly seems consistent 

 with the received views of tooth development in the Verte- 

 brata, new teeth being derived, not from the enamel organ 

 of a previously formed tooth, but from the extension 

 beyond the tooth of the tooth-band or dental lamina. In 

 Elasmobranch fishes the connexion of the newly-formed 

 teeth with a common tooth-band is very evident. As 

 C. S. Tomes says (27), in the osseous fish ' no obvious con- 

 nexion between the germs of teeth of different ages is seen ', 

 and it appears much more probable that his explanation is 

 the correct one where he says ' it is likely that the germs 

 so soon become detached that their origin from a common 

 tooth-band is masked '. 



Rose considers that in all the lower Vertebrates up to the 

 Urodelse (frogs, newts, &c.), the earliest tooth germ appears 

 as an upstanding papilla of the mucous membrane of the 

 mouth (beneath the epithelium), raised above the surface 

 of the surrounding mesodermic tissue. 



Leche denies the existence of an upstanding papilla in 

 the tooth germs of the osseous fish, but Rose considers that 

 it is easily overlooked in the very minute enamel organs of 

 fish embryos, as he has found it in all the osseous fish which 

 he has examined, as well as in reptiles. This author con- 

 siders that this indicates the origin of the jaw teeth from 



