V. THE COMPARATIVE ANATOMY OF THE INTEGUMENT AND 



THE EXOSKELETON 



A. GENERAL CONSIDERATIONS ON THE SKELETON 



The term skeleton includes all of the hardened portions of the bodies of animals. The skele- 

 ton of the invertebrates is commonly external, forming a hard covering inclosing the body, 

 while that of the vertebrates is both external and internal. In invertebrates, further, the skele- 

 ton is a lifeless secretion, containing no cells, while the vertebrate skeleton is almost invariably 

 cellular, either being composed entirely of hardened cells, or consisting of cells and intercellular 

 products. There are two distinct kinds of skeleton in vertebrates, different in origin and 

 function: (i) The external skeleton, or exoskeleton, derived from the skin, and forming a cover- 

 ing and protective layer on the outside of the body. (2) The internal skeleton, or endoskeleton, 

 derived chiefly from the inner wall of the epimere, and constituting a support and framework 

 for the body and a place of attachment of the voluntary muscles. 



We shall study the exoskeleton first. However, certain parts of the exoskele- 

 ton have become so closely related to the endoskeleton that they will be considered 

 with the latter. 



B. THE STRUCTURE OF THE SKIN 



Since the exoskeleton is derived from the skin, a thorough understanding of 

 the structure of the skin is prerequisite to a study of the exoskeleton. The skin 

 or integument occurs only in vertebrates and may be defined as those outer layers 

 of the body wall which are easily separated from the inner layers. Study of the 

 microscopic appearance and of the development of the skin reveals that it consists 

 of two distinct parts: an outer layer, the epidermis, composed of epithelial cells, 

 and an inner layer, the dermis or corium, composed of connective tissue (Fig. n). 

 The epidermis originates directly from the surface ectoderm of the embryo, which 

 by proliferation produces several strata of cells to form the epidermis. The 

 dermis is formed by the mesenchyme of the dermatome. In the preceding sec- 

 tion we learned that the dermatome originates from the outer part of the epimere 

 (Fig. 10, p. 43). 



i. Microscopic structure of the frog's skin. Examine under moderate 

 powers of the microscope a cross-section through the frog's skin and identify 

 carefully the following parts: 



a) The epidermis: The outer part of the skin consists of several strata of 

 epithelial cells. This is the epidermis. The outermost of the cell strata con- 

 sist of thin flat cells, which have become converted into a horny material. This 

 cornified part of the epidermis is designated the stratum corneum. Beneath the 

 stratum corneum the cells gradually change from a flattened to a rounded and 

 finally to a columnar shape. These lavers of rounded to columnar cells constitute 

 the stratum germinativum (also called stratum mucosum and stratum Malpighii) . In 



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