46 THE FROG 



study a portion consisting of only a single layer of cells. 

 The cells are much flattened and, as a rule, polygonal in 

 outline; each of the polygonal cells contains a nucleus near 

 its center. Among the ordinary epithelial cells of the stra- 

 tum corneum may be found occasional smaller cells with 

 rounded outline and a central tri-radiate opening; these are 

 the stoma cells, through which the mucus secreted by the 

 glands of the skin escapes to the exterior. Sketch a few 

 cells of the stratum corneum, including a stoma cell. 



B. With a hand lens study the skin of the frog, observing 

 the various colors, some of which appear better in a fresh 

 specimen. What colors do you find? Observe the small 

 papillae, formed by groups of poison glands and mucus 

 glands, found most abundantly on the dorsal surface. 



In the lighter areas along the sides of the body the black 

 pigment forms small irregular isolated spots. Cut out a 

 piece of the integument from this region, mount it on a slide 

 with a little water and cover it with a cover glass; study it 

 with the low power of the microscope. Observe that each 

 of the black patches consists of numerous black pigment 

 cells; some of these cells may be stellate in appearance and 

 their processes profusely branched, while others appear to 

 be in a much contracted condition. The black color is due 

 to a substance called melanin. Sketch a black pigment cell 

 showing numerous branches, and another in the contracted 

 condition. 



Cut out a piece of the integument from the dorsal surface 

 of the frog, mount it on a slide and study it with the low 

 power of the microscope. Observe, on focusing upon the 

 outer surface, the cells of the horny layer with here and 

 there small round transparent areas marking the location of 



