124 A MANUAL OF DENTAL ANATOMY. 



the ingrowth of the epithelium is obviously the first step 

 apparent; this ingrowth of a process of epithelium takes-, 

 place in close relation with the " neck " of an older enamel 

 organ (i.e., the contracted band of epithelium which remains 

 for some time connecting the new enamel organ with the 

 epithelium whence it was derived). New enamel organs are 

 therefore not derived directly from the epithelium of the 

 surface, but from the necks of the enamel organs of their 

 predecessors. 



In the newt, the developing teeth spread out for a con- 

 siderable distance towards the palate, and thus, being free 

 from crowding, the relations of the enamel organs of three 

 or four successional teeth of serial ages may be studied in a 

 single section j and the arrangement so disclosed may be 

 advantageously compared with that seen in the dog-fish (see 

 Fig. 57). 



The tooth sac of the newt is a good example of the sim- 

 plest form of tooth sac, consisting solely of an enamel organ 

 and a dentine germ, without any especial investment. The 

 " sac" is wholly cellular, and on pressure breaks up, leaving: 

 nothing but cells behind it. The cells of the enamel organ 

 are large, and resemble those of the eel ; the teeth of newts, 

 have a partial enamel tip, like those of the fish referred to, 

 but differing from them in being bifurcated, as is very 

 early indicated by the configuration of the enamel organ. 



In the frog there is a peculiarity in the manner in which 

 the two jaws meet, the edentulous lower jaw, which has no- 

 lip, passing altogether inside the upper jaw and its sup- 

 ported teeth, and so confining the area of tooth develop- 

 ment within very narrow limits. Consequently I have been 

 unable to satisfy myself whether the new tooth germs, or 

 rather their enamel organs, are derived from those of their 

 predecessors, or spring up de novo Analogy would indicate 

 the former, but appearances tend towards the latter sup- 

 position. 



