326 THE EX08KELET0N. 



of exoskeleton is a cuticle formed on the outer surface of the 

 epidermis, which reaches its highest development in the Arthro- 

 poda. In the same class with this cuticle must be placed the mollus- 

 can and brachiopod shells, which are developed as cuticular plates 

 on special regions of the epidermis. They differ, however, from the 

 more usual form of cuticle in their slighter adhesion to the subjacent 

 epidermis, and in their more complicated structure. The test of 

 Ascidians is an abnormal form of exoskeleton belonging to this type. 

 It is originally formed (Hertwig and Semper) as a cuticle on the sur- 

 face of the epidermis ; but subsequently epidermic cells migrate into 

 it, and it then constitutes a tissue similar to connective tissue, but dif- 

 fering from ordinary epidermic cuticles in that the cells which deposit 

 it do so over their whole surface, instead of one surface, as is usually 

 the case with epithelial cells. 



In the Vertebrata the two types of exoskeleton mentioned above 

 are both found, but that developed on the inner surface of the epi- 

 dermis is always associated with a dermal skeleton, and that on the 

 outer side frequently so. The type of exoskeleton developed on the 

 inner side of the general epidermis is confined to the Pisces, where it 

 appears as the scales ; but a primitive form of these structures persists 

 as the teeth in the Amphibia and Amniota. The type developed on 

 the O'ute?^ side of the epidermis is almost entirely^ confined to the 

 Amphibia and Amniota, where it appears as scales, feathers, hairs, 

 claws, nails, &c. For the histological details as to the formation of 

 these various organs 1 must refer the reader to treatises on histology, 

 confining my attention here to the general embryological processes 

 which take place in their development. 



The most primitive form of the first type of dermal structures is 

 that of the placoid scales of Elasmobranchii^ These consist, when 

 fully formed, of a plate bearing a spinous projection. They are consti- 

 tuted of an outer enamel layer on the projecting part, developed as a 

 cuticular deposit of the epidermis (epiblast), and an underlying basis 

 of dentine (the lower part of which may be osseous) Avith a vascular 

 pulp in its axis. The development (fig. 235) is as follows (Hertwig, 

 No. 306). A papilla of the dermis makes its appearance, the outer 

 layer of which gradually calcifies to form the dentine and osseous 

 tissue. This papilla is covered by the columnar mucous layer of the 

 epidermis (e), from which it is separated by a basement membrane, 

 itself a product of the epidermis. This membrane gradually thickens 

 and calcifies, and so gives rise to the enamel cap (0). The spinous 

 point gradually forces its way through the epidermis, so as to project 

 freely at the surface. 



The scales of other forms of fishes ai'e to he derived fronx those 

 of Elasmobranchii. The great dermal plates of many fishes have been 



I The horny teeth of the Cyclostoraata are structures belonging to this group. 

 ^ For the most important contributions on this subject from which the facts and 

 views here expressed are largely derived, vide 0. Hertwig, Nos. 306 — 308. 



