THE INTEGUMENT AND THE EXOSKELETON 81 



the skin of the embryo from a fold which involves about 

 equally both layers, and the scale develops between them, the 

 dentine being formed from the corium and the enamel from 

 the epidermis. 



In selachians the jaws are equipped with several rows of 

 pointed teeth, usually arranged like the scales which cover the 

 surface, and as the former have exactly the same embryonic 

 history as the latter and are composed of the same two layers, 

 it must be concluded that they were once simple placoid scales 

 like the rest, and that their later modifications have been due 

 to the difference of the function to which they have become 

 subjected, an inference sufficient to account for their slight 

 changes in form as well as for their increased size and hard- 

 ness, which is correlated with the greater amount of work to 

 be accomplished. These teeth, seen here almost at their point 

 of departure from generalised placoid scales, are inherited by 

 all higher vertebrates, although in some cases, like turtles and 

 birds, they have become secondarily lost. Aside from the 

 correspondence in form, arrangement and structure, the ho- 

 mology is clearly shown by the development, which proceeds in 

 all cases from a fold, involving both corium and epidermis, 

 in which the tooth subsequently appears. These organs, when 

 once acquired, are subjected to great variations as an accom- 

 modation for the prehension and mastication of the innum- 

 erable kinds of food; they develop as pointed needles, fangs 

 for inoculating poison, sharp-edged chisels, flat surfaces for 

 grinding, and ornamental tusks, in all' retaining the general 

 structure characteristic of placoid scales. The morphology 

 of the teeth will be taken up somewhat more at length in the 

 chapter on the digestive system, with which those parts be- 

 come so early associated. 



In ganoids, to which, as the lineal descendants of sela- 

 chians, one should look for the next phase of this history, the 

 scales develop from the corium alone, the epidermis remain- 

 ing passive. There is thus formed a type of scale that is com- 

 posed entirely of dentine, and lacks all trace of enamel. This 

 dentine, however, is very fine and hard in character and usu- 



