154 Iioyal Sociefi/ : — 



Tlie principal facts which my observations enable me to stato 

 are : — 



That there is no such thing as a " dental j:]n*oove " or " dental 

 fissure" in the Batnichia and Sauna, but that the whole process 

 takes place beneath au unbroken surtiU'e of epithelium. 



That there is no such thing as a stage of "■ free jiapillT," and 

 consequently no sinking of papilkc into the gum and subsequent 

 encapsulation of the same. 



Instead of being formed in a " dental groove " the teeth are de- 

 veloped in a region which may be termed the area of tooth-develop- 

 ment, varying in form and extent indifferent Kept ilia, but agreeing 

 in all in possessing the following characters : — 



It is bounded on the one side by the teeth in place and the 

 parapet of bone which carries them, and on the other, or inner, 

 side by an e.xceeduigly sharply defined boundary, consisting of 

 dense connective tissue. At the surface, near where the functional 

 tooth projects above the oral epithelium, it is narrow, but it expands 

 as it passes more deeply below the surface. Within this area are 

 developing tooth-sacs of different ages, the interspaces being 

 occupied by a loose areolar tissue, differing in appearance from 

 that which is seen outside the area, and appearing to be derived 

 from portions of older tooth-sacs, which have not been entirely 

 used up in the formation of the teeth. 



The individual tooth-sacs are formed thus : an inflection of the 

 cells of the oral epithelium, in section like a tubular gland, passes 

 down along the inner side of the area above defined, until it 

 reaches nearly to the level of the floor of the area. The depth to 

 which it penetrates is considerable in many forms, e. rj. m the 

 Lizards, in which, therefore, this double layer of epithelial cells 

 appears a mere line. 



At the bottom of this inflection of epithelial cells the adjacent 

 tissue assumes the form of a small eminence (without at first any 

 visible structural alteration), w hile the epithelial process takes the 

 shape of a bell-like cap over the eminence. 



This epithelial inflection then goes to form the enamel-organ ; 

 the eminence becomes the dentine-organ. 



Thus the enamel-germ is the first thing recognizable, and the 

 presence of this ingrowth of epithelial cells seems to determine 

 the formation of a dentine-organ at that particular spot which 

 lies beneath its tenuination. 



The enamel-organs, after they are fully formed, retain a con- 

 nexion with epithelial cells, external to the ovoid or spherical tooth- 

 sacs, at their sunnnits ; and the enamel-organs of successive teeth 

 appear to lie derived from the necks of those of their predecessors 

 rather than from fresh inflections from the surface of the oral 

 epithelium, though I am not sure that this is, in all instances, the 

 case. 



The tooth-sac of the newt is entirely cellular, and has no special 

 investment or capsule ; under pressure it breaks up and nothing 

 but cells remain, as was noted by Dr. Lionel Beale. 



