ON THE DEVELOPMENT OF TEETH. 41 



laid down in his ' Odontography ' and in his more recently 

 published works. 



According to the Goodsir theory, the first step was the 

 formation of an open groove (primitive dental groove) ; from 

 the bottom of this groove free papilla? were supposed to rise 

 uj) (dental papillae) ; Avhile their subsequent enclosure or en- 

 capsulation was believed to be brought about by an upgrowth 

 of the walls of the groove, which arched over and enclosed 

 the papilla, the development of an enamel organ being a 

 secondary process. 



More recent investigations have compelled us to adopt 

 several modifications in this account. In the first place there 

 is never an open groove at any period, and, consequently, 

 there are never any uncovered free papillae ; and, further, 

 there is no process of encapsulation, as there described. 



AVhat really takes place is this : from the deep layer of the 

 epithelium (the rete Malpighi) an ingrowth, consisting of a 

 double layer of cells, takes place, burrowing down into the 

 submucous tissue, and looking, in sections transverse to the 

 jaw, like a tubular gland (c in figs. 5 and 6). This inflection 

 of epithelium is said to take place uniformly, all round the 

 circumference of the jaw, and thus far there is no indication 

 of the position of the individual teeth (Kolliker, Waldeyer, 

 Thiersch, Frey). 



The next stage consists of an active growth of cells which 

 takes place in the deepest end of this inflection of epithe- 

 lium ; while simultaneously, or closely following upon this 

 epithelial development, the tissue immediately subjacent to it 

 becomes elevated at corresponding points, bringing about 

 the condition of things represented in fig. 2 ; these changes 

 taking place only at those spots where teeth are to be de- 

 veloped. The subjacent tissue forms a conical eminence 

 (dentine papilla, dentine pulp), which is invested above by a 

 bell-like cap of epithelium (enamel organ) — (Kolliker, 

 Waldeyer). 



It is to be noted that — 



1. There is no breach of surface, but that the whole 

 process takes place in the midst of solid tissue. 



2. The enamel organ is in no sort of way a secondary 

 formation, but that its appearance is coincident with, if not 

 antecedent to, that of the dentine papilla. 



3. There is not, and cannot be, any process of encap- 

 sulation such as was formerly described. 



The significance of these results becomes more apparent 

 when the phenomena observed in the jaws offish and reptiles 

 come to be described ; but it may be worth while to point 



