14 MICROSCOPIC ANATOMY OF THE TEETH 



separation of the cells of the mesoderm by the intrusion of 

 the epithelial band forms a groove (the primitive dental 

 groove of Goodsir), but it is not hollow, but filled with the 

 cells of this epithelial band. 



Goodsir 's views were correct up to a certain point, but 

 the methods of preparation then available did not enable 

 him to appreciate the all-important part taken by the 

 epithelial cells in this process, as these cells were washed 

 out of the depression in the mesodermic tissue produced by 

 the ingrowth of the epithelium, which then appeared as 

 a hollow groove. As pointed out by Schafer (24), Kolliker 

 showed the importance of the cells of the deeper layer of the 

 epithelium of the mouth in the production of the primitive 

 dental lamina from which the enamel organs of the teeth 

 are developed, as although attention had already been drawn 

 to this by Huxley (10) and Marcusen (17), it had not been 

 generally accepted. In fact Huxley accepted the views of 

 Goodsir, except with regard to the development of the teeth 

 in fishes and reptiles, which he did not consider could be 

 explained by this view. 



The explanation of the mode of development of the teeth 

 in the Mammalia, now generally accepted as the true one, 

 is founded upon the researches of Kolliker (12), Leche (14), 

 Rose, and many others, and has been demonstrated by 

 Rose (19) in the series of illustrative models w r hich he 

 prepared. 



There are certain differences in the development of the 

 teeth in reptiles and fishes which will be considered else- 

 where. 



At about the thirty-fourth to the fortieth day of intra- 

 uterine life, when the embryo is from 12 to 15 mm. long, 

 before the commencement of ossification, the lower jaw 

 being represented by Meckel's cartilage only, the first 

 indication of tooth formation occurs. 



This consists of an ingrowth of the deeper layers of the 

 epithelium of the mouth, forming a band which follows the 

 line of the future alveolar margin of the jaw. While it is 

 usual to speak of an ingrowth of the epithelium, it is neces- 

 sary to remember that in the embryonic jaw we are not 

 looking at a completed tissue in which the only active 



