36 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



2. The muscularis mucosae. Another difference between the stomach and 

 the intestine is that the tunica propria of the mucous membrane is sharply 

 bounded from the submucous coat by a narrow band of smooth muscle fibers 

 which is called the muscularis mucosae. It is composed of the usual two layers, 

 an inner circular and an outer longitudinal one. 



D. STRUCTURE OF THE SKIN 



Examine slide "Frog skin" (see Holmes, chap, ix, pp. 179-86). The skin 

 is a combination of epithelial and connective tissue. The epithelial part is 

 called the epidermis; the connective tissue part, the dermis or corium. 



1. The epidermis. The epidermis is a stratified epithelium, i.e., an epi- 

 thelium composed of several layers of cells. The outermost layer of the epidermis 

 (stratum corneum) consists of very flat, thin, polygonal cells which are cornified 

 or horny in composition. These cells are the ones which are shed by the frog, 

 and they have already been examined separately. Beneath the stratum corneum, 

 the cells gradually change from a flattened to a rounded, and finally to a columnar 

 shape, until the innermost ones are quite columnar in form. These layers of 

 cells constitute the stratum germinativum (also called stratum mucosum, and 

 stratum Malpighii}. The cells of the stratum germinativum frequently contain 

 brown pigment granules, and dark brown or black branched pigment cells (chro- 

 matophores] may be scattered among the regular epithelial cells. 



2. The dermis. The dermis is composed of connective tissue, separable into 

 two layers, an outer loose layer, the stratum spongiosum, and an inner compact 

 layer, the stratum compactum. 



a) The stratum spongiosum: This consists of a loose, irregular network of 

 connective tissue fibers, inclosing lymph spaces, blood vessels, etc. It contains 

 a thin layer of chromatophores, dark-colored, irregular, branching cells just 

 under the epidermis, and beneath this the cutaneous glands. 



b} The cutaneous glands: These are produced by a simple infolding of the 

 stratum germinativum of the epidermis, and their walls are a single layer thick. 

 Each opens to the surface of the skin by a narrow neck which passes up through 

 the epidermis. Explain why most of the glands on the slide appear to have no 

 neck (A). Also explain why some of them appear to be solid instead of hollow. 

 Two general varieties of cutaneous glands are recognized in the frog, the mucus 

 and the poison glands. 



The mucus glands are smaller and much more numerous than the poison 

 glands. Their appearance differs according to the stage of activity in which 

 they happen to be. In the resting or inactive state, the epithelial cells are very 

 high and conical so that the cavity of the gland is practically obliterated; the 

 nuclei are small and situated at the bases of the cells. In the active state, the 

 inner ends of the cells are converted into mucus which forms transparent masses 



