DEVELOPMENT OF THE SKIN 565 



d. Development of the Skin in the Teleost Fish 



The early development of the epidermis and dermis in the teleost embryo 

 resembles that of the shark embryo, and a soft glandular epidermis eventually 

 is formed which overlies a thick, connective-tissue-layered dermis, containing 

 numerous scale pockets, each containing a scale (fig. 264D, E). Considera- 

 tion is given next to the development of the teleostean scale. 



The development of the scale in teleost fishes is a complicated affair (Neave, 

 '36, '40). It arises in the superficial area of the dermis in relation to an aggre- 

 gation of cells. This aggregation of cells forms a dermal pocket or cavity. The 

 latter contains a fluid or gelatinous substance. The scale forms within this 

 cavity. A homogeneous scale rudiment of compact, connective-tissue fibers, 

 the fibrillary plate, is established within the gelatinous substance of the scale 

 pocket. A little later, calcareous or bony platelets are deposited upon this 

 fibrous scale plate. The scale continues to grow at its periphery and, thus, 

 stretches the dermal cavity. At the posterior margins of the scale, the dermal 

 cavity becomes extremely thin. Further growth of the scale posteriorly pushes 

 the epidermis outward, but the epidermis and the thin dermal cavity wall 

 normally retain their integrity (fig. 264D). 



The mature scale consists of a hard fibrous substrate, upon the upper pos- 

 terior margins of which are embedded calcified plates. These calcified plates 

 fuse together basally as development proceeds. Most of the scale is embedded 

 deeply in the tissue of the dermal or scale pocket. At the anterior, deeply 

 embedded end of the scale, small, hook-like, retaining barbs or teeth develop 

 along the inner margins of the scale which serve to fasten the scale within the 

 pocket (fig. 264D). 



2. Amphibia 



a. Characteristics of the Amphibian Skin 



The amphibian skin is soft, moist, and slimy. It is devoid of scales, with 

 the exception of the Gymnophiona which possess patches of small scales em- 

 bedded within pouches in the dermal layer of the skin (fig. 267J). However, 

 some of the Gymnophiona lack scales entirely. Unicellular and multicellular 

 glands of epidermal origin are a prominent feature of the amphibian skin 

 (fig. 267F, G). Specialized poison glands also are present (Noble, '31, p. 

 133). Glands are developed in some species which attract the members of the 

 opposite sex during the breeding season. In Cryptobranchus, the epidermal 

 layer may be invaded by capillaries which penetrate almost to the surface of 

 the skin in the region of the respiratory folds, located along the lateral sides 

 of the body (Chap. 14). Cornification of the outer epidermal cells is the 

 rule during later stages of development, in some species more than in others. 

 For example, the development of a cornified layer is characteristic of the 

 skin of toads, whose wart-like structures on the dorsal surface of the body 



