xiii.] THE FROG. 251 



the most part ciliated (J. i. c] ; some of the 

 papillae however will be seen to have no cilia 

 except a narrow belt around the somewhat 

 truncated apex ; it is on these latter papillae 

 that the gustatory disks are placed, and in 

 fortunate specimens nerve-fibres can be seen 

 entering 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 simi- 

 lar 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 care- 

 fully as possible, taking great care not to drag or 

 pull it. Place the fragment in 0^5^ solution of 

 silver nitrate for about three minutes : then 

 remove, wash well in distilled water, and finally 

 leave the specimen in distilled water and ex- 

 posed to the sunlight. So soon as the bit of 

 pleuroperitoneum has become of a well-marked 

 brown colour, mount it in glycerine and exa- 

 mine with a high power. 



a. 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 sil- 



