OUR FACE FROM FISH TO JNIAN 



well as the muscles of swallowing were naturally 

 of vital importance to the newborn mammal, 

 enabling it to pump the mother's milk into its 

 swelling cheeks. But how long it took mankind to 

 realize the deep significance of the fact that even 

 babies of bluest blood share this birthright with 

 the beasts of the field. 



LATER STAGES IN THE HISTORY OF THE TEETH 



Thanks to the advertisers of tooth pastes all 

 America knows the practical importance of beauti- 

 ful teeth. But few indeed share the secret as to 

 how we obtained these dazzling objects of charm, 

 and fewer still ever give a thought to the humble 

 creatures who slowly shaped them to our use. 

 It is surprising that even today, after hundreds of 

 millions of years' advancement beyond our shark- 

 like ancestors, each human being, during the 

 embryonic development of his teeth, starts at a 

 shark-like stage (Fig. 71 A). For at first the area 

 of embryonic skin that is destined to give rise to 

 the teeth lies on the surface of the mouth cavity, 

 then it sinks down like a pouch (Fig. 71B), the 

 bottom of the pouch is pushed upward (Fig. 71 C) 



to form a pulp cavity and thus the germ of the 



134 



