8o 



HISTORY OF THE HUMAN BODY 



The indifferent or generalized condition that serves as the 

 starting point for all exoskeletal elements is found in the body 

 covering of the dog-fish, which consists of imbricated rows of 

 pointed scales, that is, rows arranged in such a way that the 

 scales of one row cover the intervals of the one behind it. This 

 typical arrangement is seen also in the scales of other fishes 

 and reptiles and in the feather papillae on the skin of a plucked 



FIG. 18. Comparison in development and structure between a placoid 

 scale and a tooth. 



(a), (b), and (c) represent the scale; (d), (e), and (f) the tooth. In all the 

 figures the stratum corneum is dotted, the stratum germinativum is represented by a 

 layer of large cells with nuclei; and the cutis is presented as composed of fibers 

 with scattered cells. 



x, enamel membrane; y, cutis papilla; e, enamel; d, dentine; p, pulp cavity. 



bird. A similar, though less obvious, plan underlies the ar- 

 rangement of the hair in mammals, as will be shown later. 



The scales in the dog-fish are of the form known as placoid, 

 each consisting of an approximately flat base from which rises 

 a sharp-pointed cusp, inclined in the direction of the free edge 

 of the scale, or posteriorly when the scale is in place. This 

 scale is somewhat complex in structure and consists of a basis 

 or core of dentine overlaid by a layer of enamel, especially 

 thick over the cusp, which is almost wholly composed of it. 

 The scale is hollow beneath and a nutrient papilla formed 

 from the corium finds its way into the interior. It arises in 



