406 THE RABBIT. 



These gain admittance through a larger or smaller hole in the 

 root, or fang, of the tooth ; the hole remaining widely open 

 throughout life in the rabbit, and other animals in which the teeth 

 grow continuously throughout life, but becoming reduced to 

 one or more very small apertures in the majority of Mammals, in 

 which the teeth cease growing after they have reached their full 

 size. 



Teeth, as already noticed, are cutaneous structures; and of 

 the substances of which the tooth consists, the enamel is formed 

 from the epithelium, and the dentine and cement from the under- 

 lying connective tissue layer or dermis. 



The first step in the formation of a tooth consists in an in- 

 growth from the deeper layer of the epithelium into the con- 

 nective tissue of the gum (cf. Fig. 156). This ingrowth 

 soon becomes hollow and flask-shaped, its deeper end dilating 

 into a sac (Fig. 156, TT), while its superficial part forms a narrow 

 solid neck or stalk, which remains in connection with the surface 

 epithelium. Opposite the deeper end of the flask, the connective 

 tissue of the gum becomes condensed to form the dental papilla 

 (Fig. 156, TM). The deeper end of the epithelial flask, or enamel 

 organ as it is called (Fig. 156, TT), now becomes closely applied 

 to the dental papilla, which gradually acquires the definite shape 

 of the crown of the tooth to which it is going to give rise. 



The enamel organ (Fig. 156. TT), at this stage, is a flattened 

 sac, consisting of outer and inner epithelial layers, and having 

 its cavity occupied by a reticulum of stellate cells ; the outer 

 epithelial layer is still connected with the surface epithelium 

 by a narrow stalk or string of cells, while the inner layer 

 forms a cap, closely embracing the top and sides of the dental 

 papilla. 



This cap consists of a single layer of very regularly arranged, 

 six-sided, columnar epithelial cells ; and it is by calcification of 

 the substance of these cells that the enamel layer of the tooth 

 is produced. Calcification commences at the surface of the 

 enamel organ next to the dental papilla, and gradually spreads 

 outwards through the cells of the enamel organ. 



The dentine is formed by calcification of the dental papilla, 

 and is therefore of mesoblastic origin. Calcification appears 

 first at the surface of the papilla next to the enamel organ, so 

 that the crown of the tooth is the first part to be formed ; and, 



