DEVELOPMENT OF THE TEETH IN MAMMALIA 15 



growth is that of the epithelial cells, but at a structure 

 which is rapidly enlarging in every direction ; upgrowth and 

 downgrowth are proceeding at the same time, and the 

 tissues of the embryonic jaw are increasing in thickness as 

 well, so that the tissue near the surface becomes more 

 deeply situated, owing to the increase of growth in all 

 directions around it. 



It is thus seen that the usual description of an ingrowth 

 or downgrowth of the cells is not strictly correct, and it 

 would be more accurate to speak of it as a proliferation of 

 the deeper cells of the epithelium, which become deeply 

 situated on account of the growth of the neighbouring 

 mesodermic cells. 



Really the whole surface of the epithelium is raised, and 

 the general proliferation of cells being associated with the 

 growth of the jaw tissues on every side, the parts are in- 

 creased in size and not diminished, as might be suggested 

 by the usuaj description. To avoid confusion, however, 

 this proliferation of cells will be spoken of as an ingrowth or 

 downgrowth. 



In some mammalian germs, more especially in Ungulates, Primitive 

 there is a more or less marked heaping up of the epithelium 

 over the situation of the ingrowth. This ingrowth, the 

 'primitive dental lamina ', divides (according to Rose on 

 the forty-eighth day) into two separate bands lying 

 rectangularly to one another, the one passing perpen- 

 dicularly into the jaw (the labio-dental lamina), the other 

 more or less horizontally backwards, constituting the true 

 tooth-band or ' dental lamina '. 



The labio-dental lamina is an ingrowth of the epithelium Labio- 

 between the lip and the jaw, the cells of which atrophy and 

 form an open groove extending along the developing jaw, 

 between the tooth- band and the opening of the mouth in 

 front, and the tooth-band and the buccal wall farther back. 

 There is some difference of opinion with regard to the time 

 and mode of differentiation of the labio-dental lamina and 

 the dental lamina. These divisions of the primitive dental 

 lamina arise, according to Rose, simultaneously. Baume 

 (2) is of opinion that the dental lamina (or tooth-band) is 

 differentiated from the previously formed labio-dental lamina, 



