THE SHAPES OF ANIMAL CELLS. 



211 



and inequalities of dimensions among other aggregated cells, 

 aro caused by the equalities and inequalities of the osmotic, 

 chemical, thermal, and other forces besides the mechanical, 

 to which their different positions subject them. 



261. This we shall readily see on observing the or- 

 dinary structures of limiting membranes internal and ex- 

 ternal. In Fig. 295, is 2SS 

 shown a much-magnified 

 section of a papilla from 

 the gum. The cells of 

 which it is composed 

 originate in its deeper 

 part; and are at first 

 approximately spherical. 

 Those of them which, as they develop, are thrust outwards by 

 the new cells that continually take their places, have their 

 shapes gradually changed. As they grow and successively 

 advance to replace the superficial cells, when these exfoliate, 

 they become exposed to forces that are more and more dif- 

 ferent in the direction of the surface from what they are in 

 lateral directions ; and their dimensions gradually assume 

 corresponding differences. 



Another species of limiting membrane, called cylinder- 

 epithelium, is represented 

 in Fig. 296. Though its 

 mode of development is 

 such as to render the 

 shapes of its cells quite 

 unlike those of pavement- 

 epithelium, as the above-described kind is sometimes called, 

 its cells equally exemplify the same general truth. For the 

 chief contrast which each of them presents, is the contrast 

 between its dimension at right angles to the surface of the 

 membrane, and its dimension parallel to that surface. 



It is needless for our present purpose to examine further 



