242 ELEMENTARY BIOLOGY. [XIII. 



truncated apex ; it is on these latter papilla; 

 that the gustatory disks are placed, and in for- 

 tunate specimens nerve-fibres can be seen en- 

 tering them. 



J. SOME OF THE MORE IMPORTANT POINTS IN THE HISTO- 

 LOGY OF THE FROG. 



a. Epithelium. 



I. This consists of cells which line free surfaces within 

 the body : the epidermis covering the skin is a similar 

 structure, and is continuous with epithelium at the 

 apertures of the body. There are several main types 

 of epithelium, viz. 



a. Scaly epithelium. Open the abdomen of a re- 

 cently killed frog, carefully remove the viscera 

 and lay bare the lymph sinus at the back of the 

 body-cavity. Cut away its thin wall as carefully 

 as possible, taking great care not to drag or pull 

 it. Place the fragment in O'og solution of silver 

 nitrate for about three minutes: then remove, 

 AY ash well in distilled water, and finally leave the 

 specimen in distilled water and exposed to the 

 sunlight. So soon as the bit of pleuroperitoneum 

 has become of a well-marked brown colour, mount 

 it in glycerine and examine with a high power. 



or. It will be seen to be covered on both sides with 

 flat closely fitting cells, the boundary lines of 

 which are stained black by the silver ; accord- 

 ing to the amount of staining a nucleus may or 

 may not be rendered conspicuous in each cell. 



Here then, in good specimens, rings of 

 smaller and more deeply stained cells will be 

 seen surrounding minute apertures (stomata). 



