778 THE PROCTOD/EUM. 



the epithelium of the enamel organ and the adjoining connective tissue 

 there is everywhere present a delicate membrane known as the membrana 

 praeformativa. 



The dental papilla is formed of a highly vascular core and a non-vascular 

 superficial layer adjoining the inner epithelium of the enamel organ. The 

 cells of the superficial layer are arranged so as almost to resemble an 

 epithelium. 



The first formation of the hard structures of the tooth commences at 

 the apex of the dental papilla. A calcification of the outermost layer of 

 the papilla sets in, and results in the formation of a thin layer of dentine. 

 Nearly simultaneously a thin layer of enamel is deposited over this, 

 from the inner epithelial layer of the enamel organ (fig. 428). Both 

 enamel and dentine continue to be deposited till the crown of the tooth has 

 reached its final form, and in the course of this process the enamel 

 organ is reduced to a thin layer, and the whole of the outer layer of the 

 dental papilla is transformed into dentine while the inner portion remains 

 as the pulp. 



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 papillae 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 Verte- 

 brata except the Teleostei. In the Teleostei, however (Tomes), a fresh 

 enamel organ grows inwards from the epithelium for each successively 

 formed tooth. 



The Proctodceuni. 



In all Vertebrata the cloacal section of the alimentary tract 

 which receives the urinogenital ducts is placed in communication 



