THE DEVELOPMENT OF THE TEETH. 133 



dark halo which becomes the dentine bulb is, like the inflec- 

 tion of epithelium which forms the enamel germ, continuous 

 all round the jaw, while eventually it develops into promi- 

 nences at the points corresponding to the enamel germs of 

 future teeth, and atrophies in their interspaces. 



From the base of the dentine bulb prolongations pass out- 

 ward and slightly upwards, so that they in a measure embrace 

 the free edge of the enamel organ, and at a somewhat later 

 period they grow upwards till they fairly embrace the whole 

 enamel organ. 



These prolongations of the base of the dentine bulb are 

 the rudiments of the dental sac. In their origin, therefore, 

 the dental sac and the dentine organ are identical, and 

 spring from the submucous tissue : they contrast with the 

 enamel organ, which, as before said, is derived from the oral 

 epithelium. 



To recapitulate briefly the facts which are now established 

 beyond all question, the early mammalian tooth germ con- 

 sists of three parts, one of which, the enamel organ, is 

 derived from the epithelium of the surface ; the other two, 

 the dentine organ and the dental sac, originate in the midst 

 of solid embryonic tissue at a distance from the surface. 



The enamel organ is formed by a rapid increase of cells at 

 the bottom of a process which dips in from the stratum 

 Malpighi of the oral epithelium ; the dentine germ and the 

 dental sac are formed in close contiguity to this enamel 

 organ from the submucous tissue. 



If there was a " basement membrane " demonstrable at 

 this early period (which there is not) the enamel organ and 

 the dentine organ would lie upon the opposite sides of it. 



The description of the appearance of the several parts of 

 the tooth germ has brought us to the period at which cal- 

 cification first commences, but before proceeding further, 

 it will be well to examine more minutely the structure of 

 the several organs in which calcification takes place. 



