THE ORGANS OF THE INNEE GERM-LAYER. 309 



Whereas in the Selachians, as well as in the lower Vertebrates 

 generally, the replacement of teeth by new ones is throughout life an 

 unlimited process, since new papillae are continually being formed 

 in the depths of the dental ridge (polyphyodont), it is in the higher 

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

 There are formed on the ridge two fundaments (diphyodont), one behind 

 the other, one for the milk-teeth and a second for the permanent teeth. 



In the case of Man the development of the teeth begins as early as 

 the second month of embryonic life, A ridge (zl) (the enamel-germ of 

 older authors) grows from the epithelium of the oral cavity both 

 on the maxillary and mandibular arches as it also does in other 

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

 connective tissue. The region from which this growth into the 

 depths takes place (fig. 172 A and B) is marked exteriorly by a 

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

 dental groove (zf). The head of the human embryo represented in 

 figure 289 shows this groove at a little distance behind the fundament 

 of the upper lip. 



At first the dental ridge is uniformly thin and separated from its 

 surroundings by a smooth surface. There is nothing to be seen as 

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

 cells on the side of the riclge which is directed outwards begin at 

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

 another as many thickenings as there are to be teeth (fig. 172 A). 

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

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

 (fig. 172 fl), and gradually detach themselves from the outer surface 

 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 enamel-organs. 



In the meantime the connective tissue has taken its first steps 

 toward the formation of the tooth (fig. 172 A and .3). At the bottom 

 of each flask the connective-tissue cells exhibit active growth, and 

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

 As the papillre of the dermal teeth grow into the epidermis, so this 

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

 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 zp} 



