i io THE FROG CHAP. 



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 superficial layer of 

 the skin will be found to be 

 made up of flattened, roughly 

 hexagonal plates (Fig. 30 and 

 Fig. 27, C, D) set closely to- 



FIG. 30. Squamous epithelial cells ,., ., r 



from the frog's skin. nu. nuclei. gether, like the tlleS OI a 



mosaic pavement. Each plate 



has a nucleus, and, from its flattened form, is distinguished 

 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 form, exhibit the same 

 essential structure. Each consists of a mass of living proto- 



