CELLULAR STRUCTURE OF THE TOAD (OR FROG) 41 



the pieces of stained skin into small bits and examine one of these 

 under the microscope. 



With the low power of the microscope you will note 

 that the skin is made up of a great many flat cells placed 

 edge to edge. Each one has its cell-wall and a central 

 darkly stained nucleus. 



Make a drawing of a portion of the toad's skin. 



The liver. TECHNICAL NOTE. Cut through the fresh liver 

 of a toad, and with a knife-blade scrape from the cut surface some 

 of the liver-cells and place them on a slide with cover-glass. 



Examine under the microscope and observe many 

 polygonal cells. Place some of the methyl-green acetic 

 stain under the cover-glass and note, after the cells are 

 stained, that they have definite boundaries and a central 

 nucleus. 



Draw some of these scattered liver-cells. 



The muscles. TECHNICAL NOTE. Take a piece of intestine 

 from a freshly killed toad, wash it thoroughly and place it in a con- 

 centrated solution of salicylic acid in 70% alcohol for 24 hours, 

 then gradually heat until about the boiling-point, when the muscles 

 will fall to pieces. Transfer the preparation to a watch-crystal and 

 tease small bits of isolated muscle with dissecting-needles. Place 

 some of the teased muscle-fibres on a slide, cover with cover-glass, 

 and add a drop of the methyl-green acetic acid. 



' Note the small spindle-shaped muscle-fibres. Each 

 one of these fibres is a cell possessing all of the structures 

 common to cells, namely, cell-wall, nucleus, etc. 



Make a drawing of a few isolated fibres of muscle. 



From this study of some of the tissues in a toad it will 

 be noted that in the first case we had in the blood 

 separate cells which moved about freely in the plasma. 

 In the second case, that of the epidermis, the cells are 

 fixed edge to edge, thus forming a thin tissue; while in 

 the third and fourth cases, that of the liver and muscle, 

 the cells are not only placed edge to edge, but aggregated 



