CHAP. VII 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 
caleareous 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.’ 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),? 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, 2b. xvi. 1890, p. 97 et seq., 
p- 209 et seq. 
? Klaatsch has since affirmed the epidermic origin of the scleroblasts, ¢bid. xxi. 
1894, p. 153. 
