THE DIGESTIVE AND RESPIRATORY SYSTEM 281 



such has been undoubtedly the origin of this phenomenon, save 

 that it is not necessary to assume that all the teeth of either set 

 belong to the same original generation,- especially as they do 

 not develop simultaneously. Thus some have seen in the suc- 

 cession of mammalian teeth the remains of five separate tooth 

 generations, individuals from several uniting in a specific case 

 to form a set, either the milk or the permanent one. Others, 

 admitting that each of the two definite sets represents a single 

 tooth generation, find in certain cases traces of tooth germs 



FIG. 77. Succession of teeth in reptiles. 



(A) Section through jaw of Phyllodactylus (a lizard) showing functional tooth 

 (/) and several replacement teeth (//, ///) ; m, Meckel's cartilage; n, nerve. [After 

 GEGENBAUR.] (B) Diagram of integument of gum, to explain the succession of teeth. 

 sir. cor., stratum corneum; str. muc., stratum mucosum; /, II, etc., the succession 

 of tooth germs. 



that precede the milk dentition, and others that succeed the 

 permanent set ; they thus consider that mammals have in- 

 herited from their reptilian ancestors four tooth generations, 

 prelacteal, lacteal, permanent and post-permanent, of which 

 the second and third have become generally established.* The 

 first developed in the premammalian^ancestors, the last may 

 come to development in the future. 



Regarding the occurrence of the two dentitions there is 



* It is asserted that in Nasodon. a Tertiary ungulate, a pair of prelacteal 

 incisors becomes developed. 



