142 



STRUCTURE OF THE VERTEBRATES 



cells are released. In such cases the tooth developing cells begin 

 forming bone on top of the pulp cavity. And conversely, if den- 

 tine begins to fill the cavity, bone destroying cells normally re- 

 sorb it as rapidl}^ as it is developed. 



Other mammals have certain teeth with an open pulp cavity — 

 that is, the base remains wide open. Under these conditions the 



•Jctodermal inpocketing 



Secondary 

 vesicle 



Fig. 79. Development of Teeth (Man). Note the inpocketing of ectoderm 

 (A) to form a dental vesicle which meets the mesodermal papilla; the 

 development of a secondary vesicle and the beginning of tooth develop- 

 ment (B) ; and the growth up to the time of eruption (C) and (D). 



dentine forming cells may continue to secrete without filling the 

 inner cavity, and in that way killing the tooth. As dentine is 

 laid down on the inner side of an open cavity, the tooth is 

 pushed upward and Gontinues growing throughout life. The in- 

 cisors of rodents and several other animals continue their growth, 



