568 INTEGUMENTARY SYSTEM 



c. Development of the Skin in the Frog, Rana pipiens 



The development of the skin of the common frog resembles closely that 

 of Necturus. The primitive epidermal tube consists of two layers of ectodermal 

 cells, an outer periderm and a lower nervous layer or stratum germinativum 

 (fig. 267A). The cells of the periderm contain pigment granules, and uni- 

 cellular glands also are present, particularly in the head region. At the 10-mm. 

 stage, the outer, pigmented, peridermal layer begins to flatten, while the 

 stratum germinativum assumes the normal characteristics of the reproductive 

 stratum of the epidermis (fig. 267B). The cells are cuboidal and closely ar- 

 ranged. A condensation of mesenchyme, immediately below the thin epidermal 

 layer, represents the rudiment of the future dermis. Chromatophores are 

 prevalent in the dermal area. In figure 267C are shown the characteristics of 

 the skin of the head area of the 34-mm. tadpole, while figure 267G represents 

 the skin of the head region of the newly metamorphosed frog. In this area 

 of the body, the dermis is compact and dense, but in the dorso-lateral area of 

 the trunk, large lymph spaces are present in the dermis. 



3. Reptiles 

 a. Characteristics of the Reptilian Skin 



Most reptiles are land-frequenting animals. The land type of habitat dic- 

 tates the development of a mechanism which keeps the lower layers of the 

 epidermis soft and moist. The problem of epidermal drying is not encountered 

 to any great extent in the fishes and most amphibia because of the moist con- 

 ditions under which they live. To circumvent the drying eff'ects imposed upon 

 land-living animals, the outer layers of the skin become cornified. A super- 

 ficial or outer stratum corneum, therefore, becomes a prominent feature of 

 the epidermis of reptiles, birds, and mammals. 



Aside from its role of protecting the lower epidermal layers of cells against 

 loss of moisture, the cornified layer also functions as a protective mechanism 

 against mechanical injury. Foot pads, friction ridges, and all calloused struc- 

 tures are evidence of this function. The cornified stratum represents flattened, 

 dead, epithelial cells, infiltrated with a protein substance, keratin, present 

 abundantly in all horny structures, such as claws, scales, etc. 



Both epidermal and dermal layers are thickened considerably in reptiles, 

 while epidermal glands, so prominent in fishes and amphibia, are absent, 

 with the exception of certain specialized regions in the oral and anal areas, 

 between the carapace and plastron of some turtles, and between the scales in 

 certain areas of the skin of crocodiles and alligators. 



b. Development of the Turtle Skin 



The turtle is an example of an armored animal, possessing a "shell" con- 

 sisting of a dermal skeleton, the carapace, and the plastron, composed of a 



