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LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



the frog's skin there is no sharp demarcation between the stratum corneum and 

 the stratum germinativum. The lowermost layer of the stratum germinativum, 

 consisting of tall columnar cells, is the active portion of the epidermis and is con- 

 tinuously proliferating cells which are pushed outward, become flat and horny, 

 and finally form part of the stratum corneum. 1 The stratum corneum is shed 

 at frequent intervals and is continuously renewed from below. 



b) The dermis or corium: The dermis is the inner part of the skin. It con- 

 sists of connective tissue, which in the case of the frog is arranged partly in the 

 form of a loose network located just beneath the epidermis, and partly in the form 



gland 



FIG. ii. Diagrammatic cross-section through the vertebrate skin, based on mammals. 



of layers of dense, parallel, wavy fibers. In addition to the connective tissue 

 fibers the dermis contains : pigment cells, dark, irregular, branching cells forming a 

 thin layer just beneath the epidermis; the cutaneous glands, flask-shaped bodies, 

 produced by an infolding of the stratum germinativum, and opening by a neck 

 to the surface; and at intervals columns containing smooth muscle cells, blood ves- 

 sels, and nerves, these columns crossing the dermis at right angles to the surface. 

 Of these structures the cutaneous glands are the most conspicuous. They are 

 really parts of the stratum germLiativum, which have been evaginated into the 

 loose portion of the dermis. 



Draw a small portion of the skin to show the parts named above. 



1 In some texts only the proliferating layer, one cell thick, is called the stratum germinativum, 

 that portion of the epidermis between this and the stratum corneum being then designated the stratum 

 Malpighii. We shall here regard the terms stratum germinativum and stratum Malpighii as synony- 

 mous. 



