336 EMBRYOLOGY. 



manner, in the development of which both the superficial layer of 

 the corium and also the deepest cell-layer of the epidermis investing 

 the latter are concerned. 



(a) The corium [dermis] produces the abundantly cellular dental 



papilla, which secretes the dentine at its surface, where- 

 a layer of odontoblasts is formed. 



(b) The epidermis furnishes a layer of tall cylindrical cells, the 



enamel-membrane, which covers the dentine-cap with a 

 thin layer of enamel. 



(c) The base of the dentine-cap acquires a better attachment 



in the dermis from the fact that the latter becomes ossi- 

 fied in its vicinity and furnishes the cementum. 



5. At the margins of the jaws the tooth-forming tract of the- 

 mucous membrane sinks down into the underlying tissue ; there is 

 first developed by a proliferation on the part of the epithelium a 

 dental ridge, on which the teeth of the jaws arise in the same way 

 that the dermal teeth do on the surface of the body. 



6. The development of a tooth takes place on the ridge in the 

 following way : the epithelium grows more rapidly at one point, and 

 a papilla of the connective-tissue part of the mucous membrane 

 grows into this proliferated part or enamel-organ. The dental 

 papilla forms the dentine, but the enamel-organ, developing an 

 enamel-membrane, secretes the enamel ; finally, the connective-tissue- 

 dental sac becomes ossified and furnishes the cementum. 



7. Beneath the milk-teeth there are early formed in Mammals 

 and Man, at the deep edge of the dental ridge, the fundaments of 

 supplementary teeth. 



8. From the throat-region of the intestine there are developed 

 thymus, thyroid gland, accessory thyroid gland, and lungs. 



9. The thymus arises by the thickening and peculiar metamorphosis 

 of the epithelium of several pairs (Selachii, Teleostei, Amphibia, 

 Reptilia), or of only one pair, of visceral clefts. 



(a) In Selachians and Teleosts there is a proliferation of 



epithelium at the dorsal ends of all the visceral clefts, 

 which are penetrated by growths of connective tissue and 

 blood-vessels. 



(b) In Mammals and Man there is formed from the third 



pair of visceral clefts a pair of epithelial thymus-sacs, 

 which send out lateral buds and become peculiarly 

 altered histologically. 



(c) In Man the two thymus-sacs are joined in the median 



