76 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



These cones form simply the terminal members of a series which 

 extends inwards in the form of a curved column nearly to the inner 

 surface of the epidermis. Only the terminal members are strongly 

 cornified, the other members showing less and less cornification until 

 at a little distance down the series the cone is seen to be composed 

 of unmodified protoplasm containing at one side, near its base, a 



Pig. 45. — Illustrating the development of the larval teeth of Tadpoles. 

 A, B, C, Paludicola fuscomacuktta ; D, Sana tmivpomria. (D after Gutzeit, 1889.) 



nucleus. The cone is in i'act simply a cornified cell. Traced towards 

 the base of the column the cells are seen to be composed of more 

 granular protoplasm and to have not yet developed a hollow, while 

 the extreme base of the column is formed by an initial cell of com- 

 paratively small size and flattened shape. 



The whole column is seen, then, to be composed of a sequence of 

 conical teeth forming a replacement series, each tooth being a single 

 cornified cell. 



