2IO 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



a cap, firmly united to the top and sides of the dentine. By continued 

 additions to the deeper portions of the dentine the tooth is gradually 

 forced up through the epithelium so that its tip or crown comes into 

 position for use (eruption of the tooth). 



FIG. 210. Section of developing tooth germ of Amblystoma. e, epidermis; eo, enamel 

 organ; m, Malpighian layer; me, mesenchyme; p, pulp of tooth. 



In the lower vertebrates there may be a separate invagination of the ectoderm 

 for each tooth, but in the mammals there is a continuous ingrowth, the dental ridge 

 (fig. 211) along the margin of the jaw. Later this becomes differentiated into 

 separate enamel organs and dental papillae, the separate teeth developing much as 



FIG. 211. Model of ectodermal parts of jaw of human embryo 40 mm. long, after Rose, 

 showing the dental ridge with the enamel organs for the first teeth. 



in other groups. From the posterior side of this dental ridge there arises a continu- 

 ous projection, the dental shelf (fig. 212) which later gives rise to the enamel 

 organs for the second or permanent dentition (infra). 



The dental papilla persists throughout life as the pulp of the tooth, 

 continuing to occupy the space (pulp cavity) in which it first appeared. 



