Xiii.] THE FROG. *6i 



- outer layer frequently then getting a trans- 

 versely striated appearance and shewing a 

 tendency to split up into corresponding pieces : 

 gradually these rods entirely disintegrate, first 

 curling up, swelling out, &c. 



i. The skin. 



1. Cut out a piece of skin from the back of the thigh of 

 a recently killed frog : spread it out in water, cover, 

 and examine with a low power : note 



a. The pigment - cells ; seen as black irregularly 

 shaped patches ; some compact, others more or 

 less branched. 



b. The mouths of the cutaneous glands ; seen as 

 clear round spots, although their openings are 

 really triradiate : their number. 



2. Take a piece of skin that has lain for a day or two in 

 solution of ammonia bichromate and then in alcohol : 

 imbed it, and cut sections perpendicular to its sur- 

 faces : mount in glycerine. Examine with a low 

 power; note 



a. The two layers of the skin, dermis and epidermis, 

 the former being much the thicker : note in the 

 dermis its deeper connective-tissue layer, and 

 its more superficial granular layer immediately 

 beneath the epidermis. 



b. Examine with a higher power. 



a. The epidermis is seen to be made up of nume- 

 rous closely packed cells, arranged in several 

 layers. 



ft. The deepest epidermic cells are granular, nu- 



