DERMAL SPINES 183 



the phosphorescent organs of the deep-sea Fishes will no doubt 

 be traced to the same source. 



In the great majority of Fishes the skin becomes the seat of 

 calcareous deposit, and gives rise to such diverse exoskeletal 

 structures as the varied forms of spines and scales with which 

 the surface of a Fish is invested. 1 These structures, probably 

 the most ancient form of Vertebrate skeleton owing its existence 

 to the presence of lime salts in the tissues of the body, present 

 highly characteristic modifications in the different groups. 



Exoskeletal structures are of two kinds : ( 1 ) those which 

 owe their formation to the secretory activity of cells belonging both 

 to the epidermis and the dermis, and (2) those which are derived 

 solely from the dermis. To the first belong the dermal denticles 

 or so-called placoid scales of most Elasmobranchs, and to the 

 second the scales which form the skin-skeleton of living and 

 extinct Teleostomi and Dipnoi. With the exception of enamel, 

 which is always formed by the cells of the epidermis, the hard 

 exoskeletal tissues owe their existence to the secretion of certain 

 cells of the dermis (scleroblasts), 2 the inclusion of which in a 

 growing calcifying tissue is the cause of whatever cellular struc- 

 ture the tissue may present. It will shortly be apparent that 

 the dermic scleroblasts are by no means uniform in their pro- 

 ducts, and that in different Fishes they give rise to widely 

 different hard tissues. 



The dermal denticles or " shagreen " of the ordinary Sharks 

 and Dog-Fishes (Elasmobranchii) probably represent the most 

 primitive form of exoskeleton. In the development of a dermal 

 denticle a papilla of the dermis grows up into the overlying 

 epidermis, pushing before it the basal layer of epidermic cells, 

 which forms an investment to the papilla and constitutes the 

 so-called "enamel organ" (Fig. 100). The papilla itself sub- 

 sequently becomes converted into dentine, leaving, however, a 

 central pulp-cavity, while the apex of the papilla is invested by 

 a cap of enamel formed by the enamel organ. Ultimately the 

 base of the papilla widens out into a more or less rhomboidal 

 basal plate formed of bone. In this way there is formed a 



1 Williamson, Phil. Trans, cxxxix. 1849, p. 435 ; Hertwig, Morph. Jahrb. ii. 

 1876, p. 328 ; v. 1879, p. 1 ; vii. 1882, p. 1 ; Klaatsch, ib. xvi. 1890, p. 97 et seq., 

 p. 209 et seq. 



2 Klaatsch has since affirmed the epidermic origin of the scleroblasts, ibid. xxi. 

 1894, p. 153. 



