ELEMEKTARY STRUCTURE. 



or less rounded form (^fig. 1) ; the sphere is therefore to be consi- 

 dered as the typical form of a cell. This tendency, however, of cells 

 to assume a spherical form must be received with some limita- 

 tion, it being distinctly understood that such will be the case only, 

 when they are developed under the above conditions. But in 

 the great majority of cases, as cells are foi'med by the division 

 of older cells (as will be hereafter seen), it must necessarily 

 happen, that when first developed they will have the shape of 

 the half, the quarter, or some other section of the parent cell, 

 according to the number of parts into which it may be divided. 

 Such cells, however, if unrestricted in their after-development, 

 will then tend in the majority of cases to assume a spherical 



Fig. 1. 



Fig 2. 



Fig. 3. 



Fig. 4. 



Fig. 1. Rounded cells. 



Fig. 2. Elliptic or oblong cell.' 



Figs. 3, 4, and 5. Polygonal cells. 



form. But, in consequence of cells being usually developed in 

 a confined space, a number of other forms are produced, all 

 of which depend upon two circumstances. In the first place, 

 the form is determined by the unequal nutrition to which the 

 difierent parts of the cell-wall are subjected, thus causing a cor- 

 responding irregular growth ; and secondly, from the varying 

 pressure of the surrounding organs. We shall now allude to a 

 few of the more common forms which cells assume. 



First, when the nutrition is uniform, or nearly so, on all points 

 and sides of the cell-wall, we have a spherical or slightly ellip- 

 tic cell (Jig. 1): Avhen it is greater at the two extremities than 

 at the sides, the form is truly elliptic (Jig. 2). In the above 

 cases, also, the cells are almost free from pressure. Under 

 other circumstances, in consequence of the mutual pressure 

 of surrounding cells, they assume a polygonal form (figs. 3, 4, 

 and 5), the number of the angles depending upon the num- 

 ber and arrangement of the contiguous cells. Thus, in a per- 

 b3 



