308 



EMBRYOLOGY. 



which shares in the formation of teeth has sunk deep down in the form 

 of a ridge (zl) on the inner surface of the jaw-arches, into the under- 

 lying loose connective tissue, and now represents a special organ, 

 distinguishable from its surroundings. This important difference is 

 produced by the fact that in the development of the teeth of the jaws 

 more active processes of growth take place, first because these teeth 

 are much larger than the dermal teeth, and, secondly, because they 

 are more rapidly worn out and must consequently be more rapidly 

 replaced by supplementary teeth. As we have often had the oppor- 

 tunity of observing in the study of the production of morphological 

 conditions in animals generally, portions of epithelial membranes that 



tm zp zb 



Fig. 171. Cross section through the lower jaw of a Selachian embryo with fundaments of teeth. 

 k, Mandibular cartilage ; zl, dental ridge ; zp, dental papilla ; zb, dentine ; a, enamel ; sm, enamel- 

 membrane ; b, connective-tissue part of the mucous membrane. 



grow more rapidly than their surroundings emerge from the latter 

 and become folded either outward or'inward. 



The process of the formation of teeth is the same on the dental ridge 

 itself as upon the free surface of the skin. There are developed on its 

 outer side, which is turned toward the cartilage of the jaw (&), 

 numerous papillae (zp), lying alongside of and behind one another, 

 which grow into the invaginated epithelium just as the dermal 

 papillae grow into the epidermis. Thus there arise in the depths of 

 the mucous membrane several rows of teeth, of which the most 

 superficial anticipate in development those which lie deeper ; the 

 former are the first to break through the mucous membrane, to 

 become functional, and, after having been worn out, to be cast off; 

 they are also the first to be supplanted by reserve teeth, which lie 

 behind them, and, developing somewhat later, are consequently 

 younger. 



