DEVELOPMENT OF TEETH IN HATTERIA PUNCTATA. 179 



The teeth themselves have not yet cut the gum, but they 

 cause conspicuous projections from the surface of tlie jaws. 



In tlie lower jaw fusion with the bone is on the point of 

 taking place in some cases and has already occurred in others. 

 Tomes, as the result of the study of several reptiles (17, 18), 

 stated that the occurrence of cemeut in this class is com- 

 paratively rare. Santa Sireua (19) mentions a "falsche" 

 cement in Lacerta agilis, and in Bronn's Tierreich (20) 

 cement is described as differing from dentine in the absence 

 of dentinal tubules and the presence of "so-called" bone cells. 

 I have not succeeded in finding a satisfactoi-y account of the 

 development and structure of cement in reptiles, and Hatteria 

 in this respect confirms Tomes's results in the slowworm 

 and green lizard. Up to the stage immediately before fusion 

 the respective territories of the osteoblasts and the odonto- 

 blasts are well defined. When dentine and bone are almost 

 in contact there is a layer of flattened osteoblasts intervening, 

 and these become included in the bone or pushed to one side, 

 fusion taking place without the production of any third 

 substance whatever. The bone is very simple in structure, and 

 the dentine at the base of the tooth usuall}^ has its dentinal 

 tubules indistinctly shown, so that, as we shall see in the next 

 embryo, the transition from tooth to bone is not an abrupt 

 one as far as appearance is concerned, though the presence of 

 bone corpuscles is of course diagnostic (fig. 15). I do not 

 agree with Osawa's use of the term "osteodentine " for the 

 transitional region. 



Another ver}^ interesting feature shown in this embryo is 

 the fusion of teeth in the lower jaw. Most of the cheek 

 teeth have here begun to fuse with their anterior and pos- 

 terior neighbours, the enamel organs however apparently 

 remaining distinct from one another. The odontoblasts are 

 congi-egated in great numbers at the points of fusion. We 

 have here an undoubted case of concrescence in 

 the wider sense of the term. This process does not 

 occur, at any rate to the same extent, in any other tooth- 

 bearing region, and we shall see that it is the probable cause 



