The Inner (Alimentary) Tube and Its Respiratory Derivativi 



Fig. 18 (Left). Developing teeth of a shark, Acanthias. (e) Ingrowth of ectoderm 

 of lower jaw; (eo) enamel-organ; (m) Meckel's (mandibular) cartilage: (p) pulp 

 of (/) tooth. (Courtesy, Kingsley: "Comparative Anatomy of Vertebrates," Phila- 

 delphia, The Blakiston Company.) 



Fig. 49 {Right). Diagram of germs of milk and permanent dentitions in a 

 mammal. Based on Rose. (6) Basal layer of (e) ectoderm; (dr) dental ridge; (dtt) 

 dental shelf; (eu) enamel-organ of milk-tooth; (m) mesenchyme: (p) pulp of milk- 

 tooth: (pg) germ of permanent tooth. (Courtesy. Kingsley: "Comparative Anatomy 

 of Vertebrates." Philadelphia, The Blakiston Company.) 



Stratum germinativum 

 Oral epithelium 



Placoid scale 



Fig. 50. Diagrammatic representation of a section of a shark's lower jaw 

 showing the manner of origin of the teeth and indicating their successive replace- 

 ment. Roman numerals I— VII indicate successively older teeth. The continuous 

 movement of the teeth forward and outward is suggested by the arrows. V and VI 

 are fully developed functional teeth at the edge of the jaw. VII suggests a tooth 

 about to be shed. 



