DEVELOPMENT OF THE TEETH IN MAMMALIA 21 



the tooth-band in front of the buds for the milk teeth 

 These have often been described in developing teeth, but 

 are probably never calcified. Post-permanent teeth are 

 considered to be due to the downgrowth of the persisting 

 remains of the dental lamina or tooth-band, at the back of 

 the jaw. 



C. S. Tomes (27 a) says: 'A post-permanent set is repre- Post-per 

 sented in some animals by bands beyond the permanent ^fpre- 

 tooth germs, but these never calcify,' and he further says lacteal 

 that he ' regards both pre-milk and post-permanent rudiments 8 

 as at best hypothetical, and the evidence insufficient to 

 establish their existence ' ; but Marett Tims says : ' There 

 is now less reason for hesitation in accepting the evidence of 

 the pre-milk vestiges than was formerly the case. Doubt 

 may still exist as to the value of the post-permanent 

 downgrowths of the dental lamina.' 



In the further course of development, the successional 

 permanent teeth grow more deeply into the jaw, ingrowing 

 septa of bone separate them from the milk teeth, and they 

 come to have an alveolus of their own. 



Immediately following the formation of an enamel organ Dentine 

 from the epithelial dental lamina, the dentine germ arises papl 

 and is seen in the form of a papilla filling up the concavity 

 of the enamel organ. 



As previously stated, the production of the dentine papilla 

 seems to be- determined by the formation of an enamel 

 organ, and the dentine germ is not a true papilla, but a con- 

 densation and proliferation of the cells of the mesoderm in 

 this situation, first appearing as an opacity in the sub- 

 mucous tissue. 



Leche holds that this may be due to the crowding of the 

 cells by the surrounding growth of the epithelial cells of the 

 enamel organ. Dursy (6) considers that this opacity forms 

 a band extending all around the jaws, the prominences only 

 arising where teeth are to be formed, and the intermediate 

 portions of the opaque band becoming absorbed in the same 

 manner as are the connecting bridges of the epithelial dental 

 lamina. 



The lower and lateral margins of the dentine germ are 

 later prolonged upwards, surrounding the enamel organ and 



