THE FROG 47 



the minute stoma cells with their tri-radiate openings. In 

 the deeper layers of the skin observe the very numerous 

 black pigment cells; study this region both by reflected and 

 by transmitted light, and find patches of yellow interspersed 

 with the black pigment cells. The yellow color is due to 

 golden pigment cells, which are more or less concealed by 

 the black pigment cells. Turn the slide over and examine 

 the under surface of the skin; observe its bluish color. How 

 is the green color of the frog's skin produced? 



C. Study a prepared cross-section of the skin. Observe 

 that the skin is composed of two principal layers, an outer 

 epidermis and an inner cutis. Within the cutis is still an- 

 other layer, composed of a loose meshwork of connective 

 tissue and blood vessels, which does not belong to the skin 

 proper. In the entire frog, this layer is connected at wide 

 intervals with a similar layer of connective tissue which 

 covers the skeletal muscles, the two layers being separated 

 by great lymph spaces except where the skin is fastened to 

 the body by the septa. 



1. In the epidermis, which consists of several layers of 

 cells, observe that the outermost layer (stratum corneum) 

 is composed of extremely flattened cells. This is the horny 

 layer, described above, which is periodically shed. The 

 inner or Malpighian layer is composed of several layers of 

 cells; the innermost of these consists of columnar cells, the 

 other layers show transitional stages between the latter and 

 the stratum corneum. 



2. The cutis is composed mainly of connective tissue in 

 which are imbedded numerous glands ; it is roughly divisible 

 into the following two layers : 



(a) The outer layer, composed mainly of loose connec- 



