110 



THE FROG 



CHAP. 



whip-lashes, or, more accurately, like the blades of 

 grass in a field when acted upon by a strong wind. If 

 you happen to get under the microscope a good-sized 

 bit of mucous membrane with the cells in position, you 

 will see that the cilia produce a strong current by which 

 small particles are swept along, while detached cells 

 swim about, like little independent animals, by the 

 action of their own cilia. These ciliated epithelial cells, 

 like the ordinary columnar cells of the intestine, are 

 made of protoplasm, and each contains a nucleus with 

 one or two nucleoli clearly brought into view by staining. 

 The action of the cilia can be demonstrated, on a large 

 scale, by placing a freshly-killed frog on its back, turning 

 back or cutting away the lower jaw, and placing a very 

 small cube of cork on the roof of the mouth near to the 

 projection due to the eyes. The cork will be slowly 

 swept back towards the throat. 



Squamous or Pavement Epithelium. By scraping 

 the outer surface of a piece of skin with a sharp knife, 



and examining the scrapings 

 in a drop of water, after 

 staining them, the super- 

 ficial layer of the skin will be 

 found to be made up of 

 flattened, roughly hexa- 

 gonal plates (Fig. 30 and 

 Fig. 27, C, D) set closely 

 together, like the tiles of a 

 mosaic pavement. Each 



plate has a nucleus, and, from its flattened form, is dis- 

 tinguished as a squamous or scale-like epithelial cell. 



Meaning of the word " Cell." We see thus that 

 the body of the frog is partly made up of distinct elements, 

 which, under a considerable diversity of forms, exhibit 

 the same essential characters. Each structure consists 



FIG. 30. -Squamous epithelial cells 

 from the Frog's skin, (x 300.) 

 nu nuclei. 



