The Inner (Alimentary) Tube and Its Respiratory Derivatives 39 



contains no living substance and is incapable of self-repair, but loss of 

 dentine at the outer surface of the layer may be compensated for by 

 deposition of additional dentine at the internal 

 surface. 



Development of Teeth 



Teeth develop in much the same way in 

 all vertebrates, but the process is necessarily 

 more complex in the larger and more highly 

 differentiated teeth. Naturally, the develop- 

 ment of human teeth has been especially care- 

 fully studied. A brief account of it may well 

 serve the present purpose. 



When the human embryo has attained a 

 length of about 11 mm. — that is, by the end 

 of the sixth week of development — the ecto- 

 dermal epithelium covering the jaws grows 

 rapidly into the underlying connective tissue 

 to form adental ridge or lamina extending 

 parallel to the edge of each jaw. The lamina is 

 formed by multiplication of cells in the deep- 

 est layer (stratum germinativum) of the 

 ectodermal epithelium which is continuous 

 with the external epidermis (Fig. 35). As 

 growth continues, the deep edge of the lamina 

 (labial) lamina and an inner (lingual) lamina. 



0, 



Fig. 34. Five odonto- 

 blasts, from which 

 Tomes's fibers extend up- 

 ward into the dentine; 

 from a tooth of a new- 

 born cat. (After Prenant. 

 Courtesy, Bremer: "A 

 Text-Book of Histology," 

 Philadelphia, The Blakis- 

 ton Company.) 



divides into an outer 

 With further growth, 



C0RIUM 



BONY ALVEOLUS 



LINGUAL LAMINA 



ENAMEL ORGAN OF PERMANENT TOOTH 



Fig. 35. Diagrams of three stages in the development of a mammalian tooth as 

 seen in sections of the jaw. The enamel-organ of the permanent tooth lies on the 

 lingual side of that of the milk-tooth. (After O. Hertwig and Arey. Courtesy, Neal 

 and Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



