246 



THE DIGESTIVE ORGANS. 



tiva), the axial portion of the cells remaining intact as the dentinal 



fibers ; the basal portions of the cells containing the nuclei persist, 



later constituting the odontoblasts of the adult pulp. By the fusion 



of the segments of the ground-substance formed by each cell, it 



becomes a homogeneous mass, but soon displays connective-tissue 



fibrils which gradually undergo a process of calcification. The mem- 



., brana praeformativa has 



" no fibers and calcifies 



a ^ 



*L<% .~w.^ much later. It lies im- 



mediately beneath the 

 enamel or the cementum, 

 and in the normal tooth 

 always contains small in- 

 terglobular spaces. In 

 the adult tooth this mem- 

 brane in its entirety is 

 a known as Tomes' gran- 



ular layer. 



fe- b 



"~~~ Enamel pulp. 



Enamel cells. 





I . i v .- IB 



& 







tv 



ji Odontoblasts. 



--: C 



Fig. 190. A portion of a cross-section through 

 a developing tooth (later stage than in Fig. 189) ; 

 X 720 : The dentin is formed, but has become homo- 

 geneous from calcification. Bleu de Lyon differen- 

 tiates it into zones (a and b}. At c is seen the in- 

 timate relationship of the odontoblasts to the tissue of 

 the dental pulp. 



The cementum is 

 merely a periosteal 

 growth of bone originat- 

 ing in the tissue of the 

 dental sac and adhering 

 to the dentin. Although 

 at first the enamel or- 

 gan almost entirely sur- 

 rounds the dentinal pa- 

 pilla, later a portion of 

 that part of it in the re- 

 gion of the fang is ab- 

 sorbed in order to allow 

 the cementum to reach 

 the surface of the dentin. 



Remains of this regressive portion persist as the epithelial nests of 

 the dental root (compare p. 242). 



The contents of the dentinal papillae change into the tissue of the 

 dental pulp. 



As early as, the third month outgrowths appear on the inner 

 side of the enamel ledge next to the partly developed milk-teeth, 

 which represent the anlagen of the enamel organs of the permanent 

 teeth. Their further development is similar to that of the milk teeth. 

 The enamel organs of the molars are also developed from an enamel 

 ledge which is practically a backward continuation of the embryonic 

 enamel ledge. With their crowns presenting, the temporary teeth 

 at last break through the epithelium of the gums. When the de- 

 velopment of the permanent teeth is so far advanced that they are 

 ready to perforate, regressive processes begin at the roots of the 



