INTRODUCTION 557 



which overlies muscles, bones, and tendinous structures of the body (fig. 



272H). 



2. General Functions of the Skin 



The integument acts as a barrier between other body tissues and the ex- 

 ternal environment. Modifications of the integument serve also as an external 

 skeleton or exoskeleton in many vertebrates. In warm-blooded forms, the skin 

 is associated intimately with the regulation of body temperature. The hypo- 

 dermal portion of the skin often serves to store reserve fatty substances. The 

 presence of fat functions as a buffer against mechanical injury from without, 

 as reserve food, and as an aid in temperature regulation in warm-blooded 

 species. Still another and very important function of the skin is its intimate 

 association with the end organs of the peripheral nervous system by means 

 of which the animal becomes acquainted with changes in the external environ- 

 ment. (See Chap. 19.) 



3. Basic Structure of the Vertebrate Skin in the Embryo 

 a. Component Parts of the Developing Integument 



In all vertebrates, the integument arises from a primitive embryonic integu- 

 ment which at first is composed of the cells of the epidermal tube only, i.e., 

 the primitive epidermis. Later this rudimentary condition is supplemented 

 by a condensation of mesenchymal cells below the epidermis. Following this 

 contribution, the primitive skin is composed of two main cellular layers: 



(1 ) a primitive epidermal (ectodermal) layer of one or two cells in thick- 

 ness and 



(2) an underlying mesenchymal layer. 



The former gives origin to the epidermis, while the latter is the fundament 

 of the dermis. A little later, chromatophores or pigment cells, presumably of 

 neural crest origin, wander into the primitive dermis and become a con- 

 spicuous feature of this layer. In the development of the vertebrate group as 

 a whole, these two basic layers serve as the basis for the later development of 

 the integument. As a result, these two layers undergo characteristic modifi- 

 cations which enable the skin to fulfill its specific role in the various vertebrate 

 species. The marked differences in later development of these two integumen- 

 tary components in different vertebrate species are associated with the needs 

 and functions of the skin in the adult form. 



b. Origin of the Component Parts of the Early Integument 



1) Origin of the Epidermal Component. The epidermal component de- 

 scends directly from the primitive epidermal (ectodermal) organ-forming 

 area of the late blastula, which, as we have seen, becomes greatly extended 



