THE TEETIT. 



107 



horny investment the cuticula (membrane of the enamel) or 

 it becomes elongated in a remarkable manner into long, petri- 

 fied prisms, which collectively invest the dentine and are 

 known as the enamel." 



Preparations for the development of the teeth take place at 

 a time when the epithelium of the mucous membrane of the 

 mouth is found growing downward, like a solid peg, with a 

 rounded extremity. 



This has been called the primary enamel organ. As a next 

 step, the material which is to give form to the tooth pushes 

 upward as a papillary growth, and meeting 

 the epithelial peg, pushes in or invaginates 

 its rounded extremity. This is the tooth 

 papilla, and as it pushes upward the pri- 

 mary enamel organ becomes the secondary 

 enamel organ, or the enamel cap. We have 

 now two tissues which are embedded in the 

 soft embryonic substance, that happens at 

 this early period to be gelatinous. That 

 portion of it immediately surrounding the 

 papilla and cap is called the tootJi-sac. 



The papilla, which becomes highly vas- 

 cular, is covered, on its outer surface, by 

 the odontoblasts, a layer of columnar epi- 

 thelial corpuscles, which elongating, are 

 transformed directly into the dentinal sub- 

 stance at their outer extremity. 



According to Kolliker and others, they 

 excrete the dentine. The former view seems 

 to have the most weight of argument in its 

 favor, but it seems less likely that the odon- 

 toblasts both make the matrix and send 

 fibres into the tubulse. The view of Klein 

 already given seems to be preferable, and in conformity with 

 what we know of other connective substances. 



The separation of the tooth-sac from the mucous membrane 

 is effected by the gelatinous tissue, which, gradually closing in 

 the neck of the sac, finally cuts it off. The epithelium of the 

 enamel cap is abundant and of various kinds; into it push a 

 number of papillary processes downward from the gelatinous 

 tissue. Later the enamel cap is changed into three membranes. 



Fia. 42. Longitudinal sec- 

 tion of a milk tooth from the 

 foetal sheep, carried through 

 the margin of the dentine 

 pulp and adjoining portion of 

 the enamel organ. Magni- 

 fied 200 diameters. 1, dental 

 sacculns; 2, external epithe- 

 lium and stratum interme- 

 dium here united to the in- 

 ternal epithelium or enamel 

 cells ; 3, after the disappear- 

 ance of the enamel pulp ; 4, 

 young layer of enamel de- 

 tached from the enamel cells ; 



6, dentine; 6, odontoblaata; 



7, part of the dentine pulp. 

 (Waldeyer.) 



