562 INTEGUMENTARY SYSTEM 



The dermal layer of fishes is a fibrous structure of considerable thickness. 

 The layer of dermal tissue, immediately below the epidermis, is composed of 

 loosely woven, connective-tissue fibers, copiously supplied with blood vessels, 

 mesenchymal cells, and chromatophores. Below this rather narrow region is 

 a thick layer, containing bundles of fibrous connective tissue. Between the latter 

 and the muscle tissue is a thin, less fibrous, subcutaneous layer (fig. 263E). 



Scales are present generally throughout the group and are of dermal origin 

 in most species. However, both layers of the skin contribute to scale formation 

 in the shark and ganoid groups of fishes. Scales are absent in some fishes as, 

 for example, in cyclostomes and certain elasmobranchs, such as Torpedo. In 

 certain teleosts, the scales are minute and are embedded in the skin. This 

 condition is found in the family AnguiUidae (eels). 



Highly specialized, phosphorescent organs are developed in deep-sea fishes 

 as ingrowths of masses of cells from the epidermis. (Consult Green, 1899.) 

 These epidermal ingrowths (fig. 265 A) separate from the epidermal layer and 

 become embedded within the dermis (fig. 265B, C). 



b. Development of the Skin in the Embryo of the Shark, Squalus 



acanthias 



1) Epidermis. In shark embryos up to about the 15-mm. stage, the integu- 

 ment consists of an epidermis composed of one layer of cells, one cell in 

 thickness (fig. 263A). The shapes of these cells may vary, depending upon 

 the area of the body. In some areas, especially the dorso-lateral region of the 

 trunk, they are flattened, while along the middorsum of the embryo they are 

 cuboidal. In the pharyngeal area they are highly columnar. 



By the time the embryo reaches 25 to 35 mm. in length, two layers of cells 

 are indicated in the epidermis, an outer periderm of much-flattened cells and 

 a lower, basal, germinative layer, the stratum germinativum (fig. 263B). The 

 stratum germinativum retains its reproductive capacity throughout life, giving 

 origin to the cells which come to lie external to it. Eventually the epidermis 

 is composed of a layer of cells, several cells in thickness. The outer cells may 

 form a thin squamous layer, covering the external surface (fig. 263D). 



2) Dermis. The dermis gradually condenses from loose mesenchymal cells 

 which lie below the stratum germinativum of the epidermis (fig. 263B, C). 

 The dermis gradually increases in thickness and becomes composed of scat- 

 tered cells, intermingled with connective-tissue fibers. Deeply pigmented chro- 

 matophores become a prominent feature of the dermal layer, where they lie 

 immediately below the germinative stratum (fig. 263D, E). 



3) Development of Scales and Glands. In the formation of the placoid scale 

 of the shark, masses of mesenchymal cells become aggregated at intervals 

 below the stratum germinativum to form scale papillae (fig. 263C). Each 

 papilla gradually pushes the epidermis outward, especially the basal layer (fig. 

 263D). The cells of the outer margin of the papilla give origin to odontoblasts 



