480 STRUCTURE AND DEVELOPMENT OF TEETH, W. WALDEYER. 



longitudinal furrow, with rounded borders, termed the " dental 

 groove." The epithelium of the oral cavity completely covers 

 it, so that it is scarcely perceptible when the surface alone is 

 examined. The two projecting borders of the groove are 

 termed the " dental ridges " (Marcusen, 31), or " lips of the 

 dental groove" (Dursy, 67). Soon, from the bottom of the 

 dental groove, a narrow process of the oral epithelium dips 

 into the subjacent mucous tissue, presenting on section the 

 form of a short tubular gland, but in point of fact constituting 

 an epithelial fold along the whole length of the jaw the 



Fig. 100. 



Fig. 100. Upper jaw of a foetal sheep three centimeters in length. 

 Vertical section, magnified 50 diameters, showing the enamel germ, 

 with the semi-lunar rudiment of the dentine germ and dental sac in 

 transverse section. 1. Dentinal groove. 2. Palatal process. 



enamel germ of Kolliker (47). The primary dental groove, 

 especially of the upper jaw, increases in size, and becomes 

 entirely filled with oral epithelium. The epithelium also 

 becomes extraordinarily increased in thickness on the two 

 dental ridges, and in the deep groove between the lips and the 

 margin of the jaw, especially in Ruminants (Kolliker, 47). 

 At some points the enamel germ appears to descend perpen- 

 dicularly from the base of the furrow into the subjacent tissue, 

 but in other regions, especially in the neighbourhood of the 

 incisors, it extends obliquely towards the median line, and 

 consequently forms a larger or smaller angle with the dental 

 groove. 



