The root of the tooth is formed later than the crown, but the enamel organ is not prolonged over this part, so that it is only formed of dentine.
By the formation of the root the crown of the tooth becomes pushed outwards, and breaking through its sack projects freely on the surface.
The part of the sack which surrounds the root of the tooth gives rise to the cement, and becomes itself converted into the periosteum of the dental alveolus.
The general development of the enamel organs and dental papillæ is shewn in the diagram ([fig. 428]). From the epithelial ridge three enamel organs are represented as being developed. Such an arrangement may occur when teeth are successively replaced. The lowest and youngest enamel organ (e) has assumed a cap-like form enveloping a dental papilla, but no calcification has yet taken place.
In the next stage a cap of dentine has become formed, while in the still older tooth this has become covered by a layer of enamel. As may be gathered from this diagram, the primitive epithelial ridge from which the enamel organ is formed is not necessarily absorbed on the formation of a tooth, but is capable of giving rise to fresh enamel organs. When the enamel organ has reached a certain stage of development, its connection with the epithelial ridge is ruptured ([fig. 428]).
The arrangement represented in [fig. 428], in which successive enamel organs are formed from the same epithelial ridge, is found in most Vertebrata except the Teleostei. In the Teleostei, however (Tomes), a fresh enamel organ grows inwards from the epithelium for each successively formed tooth.
The Proctodæum.
In all Vertebrata the cloacal section of the alimentary tract which receives the urinogenital ducts is placed in communication with the exterior by means of an epiblastic invagination, constituting a proctodæum.
This invagination is not usually very deep, and in most instances the boundary wall between it and the hypoblastic cloaca is not perforated till considerably after the perforation of the stomodæum; in Petromyzon, however, its perforation is effected before the mouth and pharynx are placed in communication.
The mode of formation of the proctodæum, which is in general extremely simple, is illustrated by [fig. 420] an.