ORGANS OF Till I .'509 



"Whereas in tli- Selachians, as well as in tin* ! rtebrates 



generally, tin- rtplacemtnt <>/ t-th A// new ones is throughout life an 



vnlunit"! proces>. since new papilhe are continually In-ing formed 



in tin- depths of tin- dental ridgo (polyphyoda in tli" higher 



Vertebrates more limited, ami in most Mammals occurs only once. 



a r>\ f,- in f on th- .-/</>/' tim fundaments (diphyodont ). <,,/,' /W/////7 



t1i>T. one for the milk-t<>>'tJi and a second for the permanent teeth. 



In f/f case of Man the dt'>-> !,,/,,,,,,,( ,,/' /// as 



the second mnnth of an fir </<>///<' /if',-. -ni of 



older aut hoist grows from the epithelium of the oral cavity both 

 on the maxillary and mandibular arches as it al>o doe< in other 

 mammalian embryos (fig. 290) into the richly cellular embryonic 

 connective tissue. The region from which this growth into tin- 

 depths takes place (tig. 172 A and /?) is marked exteriorly by a 

 \e, which runs parallel to the arch of the jaw, the so-called 

 dental groove (zf). The head of the human embryo rc|>iv> -nted in 

 figure 289 shows this groove at a little distance behind the fun-la' 

 of the upper lip. 



At first the dental ridire is uniformly thin and l 

 surroundings hv a smooth surface. There is nothing to be seei 

 yet of the separate fundaments of the teeth. Then the epithelial 

 cells on the side of the rid*,'!' which is directed out \\ards beijin at 

 certain places to grow and to produce at regular intervals from one 

 another as many thickening re to be teeth (lig. 172 A). 



In Man. who has twenty milk-teeth, the number of these is ten 

 in each jaw. The thickenings now assume a llask-shaped form 

 (fig. 172 /?), and gradually detach themselves from the outer su: 

 of the epithelial ridge (zl), except at the neck of the flask, which 

 remains in connection with it at a little distance from its deep edge. 

 Because these epithelial growths have relation to the secretion of 

 enamel, they have received the name of e> 



In the meantime the connective tissue has taken its first steps 

 toward the formation of the tooth (fig. 172 A and B). At the bottom 

 of each tlask the connective-tissue cells exhibit active srrowth, and 

 give rise to a papilla (zp) corresponding in form to the future tooth. 

 As the papillae of the dermal teeth grow into the epidei -mis. so this 

 papilla grows into the enamel-organ, which is thereby made to take 

 the form of a cap. 



Then the special layers from which the formation of dentine and 

 enamel proceed are differentiated in both fundaments so far as these 

 are in mutual contact. At the surface of the papilla (fig. 172 B zj>) 



