THE TEETH 



331 



During the third month of fetal life the anlages of all primitive 

 (deciduous, mill?,, or temporary} teeth are formed in the above manner. 

 At about the same time, also, a posterior growth upon the lingual side 

 of the thin portion of the dental shelf which still connects the enamel 

 germs with the oral epi- . ^ ^ ./.-; 



thelium, forms the anlages yXv/'^V*/S?vX*-%:X .-. . 



of twenty of the permanent 

 teeth. The twelve addi- 

 tional permanent molars 

 arise at a later period and 

 in a similar manner by a 

 dorsal extension of the 

 dental lamina which grows 

 backward through the con- 

 nective tissue of the alveo- 

 lar process as a solid cell 

 column from which the en- 

 amel germs are formed 

 and into which the dental 

 papillae grow. 



Further development 

 of the dental anlage in- 

 cludes the differentiation 

 of the enamel germ on the 

 one hand and of the dental 

 papilla on the other. From 

 the former the enamel and 

 the cuticular epithelial 

 membrane arise ; the latter 

 produces the dental pulp 

 and the dentin. 



THE ENAMEL GERM. 



FIG. 311. DEVELOPING TOOTH FROM A HUMAN 

 EMBRYO 17 MM. LONG. 



LF, dental groove; M, oral cavity; OK, meso- 

 blast of upper jaw; UK, anlage of lower jaw; OL, 

 epithelium of the primitive upper lip, and UL, 

 of the lower lip; ZL, dental lamina (labiodental 

 strand). X 120. (After C. Rose.) 



The enamel germ or 



enamel organ soon differentiates into three layers: (1) an inner enamel 

 epithelium which forms the enamel prisms; (2) an outer enamel epithe- 

 lium which lines the dental sac ; and ( 3 ) an intervening enamel pulp. 



THE INNER ENAMEL EPITHELIUM. The innermost cells of the 

 enamel organ, viz., those which rest directly upon the dental papilla, 

 soon become elongated and attain a cylindrical form. The nucleus 

 moves toward the distal pole, and the original basal end becomes modi- 



