THE SHAPES OP ANIMAL CELLS. 



229 



and inequalities of dimensions among other aggregated cells, 

 are 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 ordinary 

 structures of limiting membranes, internal and external. 

 In Fig. 295, is shown a 29s 



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 which are more and more different 

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

 directions; and their dimensions gradually assume corre- 

 sponding 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 



