VERTEBRATE ANIMALS 213 



embedded in the upper part of the dermis, while the spine, which is 

 generally leaf-shaped with a long point flanked on each side by a 

 shorter one, projects through the epidermis. The greater part of 

 the scale consists of hard calcareous substance, dentine, a dermal 

 product, covered with a layer of still harder enamel secreted by the 

 epidermis. The denticle is hollow, containing within it a pulp 

 cavity in the form of a median canal with side branches which opens 

 to the outside by means of a more or less circular hole in the basal 

 plate. In life this cavity is filled with a loose connective tissue, the 

 pulp, containing blood-vessels, lymph-vessels, and nerves which 

 pass in through the hole in the base. The pulp is surrounded by a 

 single layer of characteristic cells known as odontoblasts, the function 

 of which is to secrete the dentine. This dentine, unlike bone, contains 

 no cells within it, and therefore grows only on the side next to the 

 odontoblasts. Its matrix contains a very high percentage of 

 calcium salts, considerably more than in bone, and is penetrated by 

 a number -of extremely fine processes from the odontoblasts which lie 

 in tiny branched dentinal tubules. The enamel or vitrodentine, as 

 it is termed, has a still higher proportion of calcium salts, and is in 

 consequence harder. Although it does not possess the prismatic 

 structure characteristic of the enamel of our own teeth, it is probably 

 homologous with it. 



It will be remembered that the lining of the buccal cavity is 

 essentially the same as the skin covering the body, and, although 

 most of it is devoid of exoskeletal structures, as it passes over the 

 jaws it produces a series of rows of highly modified denticles, the 

 teeth. These have the same fundamental structure as the ordinary 

 scales, which owe their name of denticles to this similarity, and from 

 which they have undoubtedly been derived. The blade, however, is 

 broader and not so long, and the median spine has two or three 

 spines on each side of it, all being of approximately the same length. 

 The enamel layer is also considerably thicker. Various modifica- 

 tions of the denticles produce the characteristic spines and teeth 

 met with in all the Elasmobranchs. 



The development of a denticle calls for notice owing to its 

 similarity with that of our own teeth. The first indication of the 

 formation of a scale is the aggregation of a number of the cells of the 

 superficial layer of the dermis to constitute a dermal papilla, and this 

 represents the origin of the pulp. This papilla enlarges and presses 

 upwards into the epidermis, the Malpighian layer of which becomes 

 modified to form a layer of columnar cells, the enamel epithelium. 

 While this is taking place, the outermost cells of the papilla become 

 transformed into odontoblasts which secrete the dentine, first in the 

 form of a small cone capping the papilla. The base of the papilla 



