558 



INTEGUMENTARY SYSTEM 



during gastrulation and, in the post-gastrular period, is tubulated into the 

 elongated, cylinder-like structure. The primitive epidermal tube thus forms 

 the initial skin or outer protective investment of the developing body. 



The wall of the primitive epidermal tube at first may be composed of a 

 single layer of cells of one cell in thickness, as in the shark, chick, pig, opossum, 

 or human (figs. 263 A; 269A; 272A). However, in teleost fishes and amphibia, 

 the primitive epidermal tube is composed of two layers of cells. For example, 

 in the sea bass, the wall of the primitive epidermal tube is composed of two 

 layers, the outer layer being thin and made up of much-flattened cells and 

 the lower layer being two cells in thickness (fig. 264A, B). In the anurans 

 and urodeles, the wall of the primitive epidermal tube is composed of two 

 layers, each of one cell in thickness (fig. 267 A, D). The lower layer in the 

 frog, salamander, anjd teleost often is referred to as the inner ectodermal or 

 nervous layer. It is the germinative layer and thus forms the inner or lower 

 portion of the stratum germinativum of the later epidermis (fig. 267 A, D). 

 The outer layer is densely pigmented and forms the periderm. 



In the embryo of the shark, chick, and mammal, the single-layered condition 

 of the primitive epidermal tube soon becomes transformed into a double- 

 layered condition, the outer layer or periderm being composed of much- 

 flattened cells (figs. 263B; 269B; 272B). In all vertebrates, therefore, the 



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Fig. 263. Developing skin of Sqiialus acanthias. (A) Section through differentiating 

 somite and epidermis of 10-mm. embryo. (B) Integument of 34-mm. embryo. (C) 

 Section of skin, showing beginning of scale formation in 60-mm. embryo. (D) Scale 

 development in 145-mm. embryo. (E) Later stage of placoid scale, projecting through 

 epidermal layer of skin. 



