VI PARENCHYMA CELLS 59 



elongated Amoeba without pseudopods. In some animals 

 the resemblance is still closer : the epithelial cells have been 

 observed to throw out pseudo])ods from their free surfaces 

 (d^), that is, from the only part where any such movement 

 is possible, since they are elsewhere in close contact with 

 their fellow cells. 



The mouth of the frog and the trachea or windpipe of air- 

 breathing vertebrates such as reptiles, birds, and mammals, 

 are also lined w-ith mucous membrane, but the epithelial 

 cells which constitute its inner layer differ in one important 

 respect from those of the stomach and intestine. If ex- 

 amined quite fresh each is found to bear on its free surface, 

 i.e.^ the surface which bounds the cavity of the mouth or 

 windpipe, a number of delicate protoplasmic threads or 

 cilia (e^ — E^) which are in constant vibratory movement. In 

 the process of teasing out the mucous membrane some of 

 the cells are pretty sure to become detached, and are then 

 seen to swim about in the containing fluid by the action 

 of their cilia. These ciliated epithelial cells remmd one 

 strongly of Heteromita, except for the fact that they bear 

 numerous cilia in constant rhythmical movement instead of 

 two only — in this case distinguished as flagella — presenting 

 an irregular lashing movement. 



Similar ciliated epithelial cells are found on the gills ot 

 oysters, mussels, &c., and in many other situations. 



The stem or root of an ordinary herbaceous plant, such 

 as a geranium or sweet pea, is found when cut across to 

 consist of a central mass of pith, around which is a circle 

 of woody substance, and around this again a soft greenish 

 material called the cortex. A thin section shows the latter 

 to be made up of innumerable polyhedral bodies called 



