122 



THE BIOLOGY OF THE FROG 



CHAP. 



An excellent example of flattened or squamous epithelium 

 may be obtained in the outermost skin which is cast off 

 during the molt. The cells of this layer are broad and ex- 

 ceedingly thin, and show a rounded 

 nucleus near the center. The cells 

 of the peritoneum are mostly of the 

 same flattened type. In the colum- 

 nar epithelium the cells are elon- 

 gated perpendicularly to the sur- 

 face and are usually prismatic in 

 outline, owing to mutual pressure ; 

 such epithelium is common in the 

 FIG. 31. A portion of the mucous layer of the intestine. In 

 epidermis of Rana pipicns. mail y places, as in the outer skin, 



s, stoina cell. . . 



there may be all transitional stages 



between columnar epithelium and squamous epithelium.. 

 Layers such as this which are several cells deep are called 

 stratified epithelium. 



In some parts of the body there occurs a peculiar variety 

 called ciliated epithelium in which the cells are furnished 

 with cilia at their outer ends. Usually such cells are colum- 

 nar, but they may be cuboid or even somewhat flattened. 

 Ciliated epithelium occurs in the mouth and throat of the 

 frog, in certain parts of the peritoneal lining of the body 

 cavity, on the inner lining of the oviducts, in the mouths 

 of the ciliated funnels of the kidney, in the ventricles of the 

 brain, and, in early larval life, on the outer surface of the 

 body. If the roof of the mouth of a frog be scraped with 

 a knife and the cells removed and examined under a micro- 

 scope, a shimmering movement may be seen on one side of 

 each cell. This is due to the rapid movement of the cilia or 

 fine hairlike processes on the surface. The cilia of all the 

 cells of a particular area beat most strongly in one direction, 



