THE CELL DOCTRINE. 145 



distinct than the intracellular. See plate at end of 

 volume. 



TTie Shape of Cells. — The shape of cells is extremely 

 varied. In its fully developed state each tissue is com- 

 posed of cells which may be said to be characteristic of 

 it. First, the typical form of a cell may be said to be 

 spherical, and this is the shape of all young cells in 

 whatever situation found. See Fig. 10, frontispiece. 

 Second, in glandular tissue, where perhaps cells are 

 changing more rapidly than in any other situation 

 in health, their deviation from the round shape is 

 only such as results from mutual compression, forming 

 more or less polygonal cells, as seen in Fig. 3, p. 39. 

 ITiird, in epithelial structures we have upon the ex- 

 treme periphery either flat scales containing a small 

 proportion of bioplasm (nucleus) in proportion to the 

 non-germinal matter (cell-contents), and of irregular 

 outline, forming the so-called squamous epithelium. 

 Figs. 3, 4, 5, 6, frontispiece; or fourth, similar scales, 

 of regular outline, many-sided and with their edges 

 so adapted as to form a pavement-like structure, 

 forming tessellated epithelium ; or fifth, elongated nu- 

 cleated cells, which from their shape are called colum- 

 nar cells. Sixth, in certain situations, as the respira- 

 tory passages, these columnar cells are further provid- 

 ed with hairlike prolongations, known as cilia, which 

 in health exhibit a constant waving motion, whence 

 the cells are called ciliated cells. Wherever columnar 

 cells are present in successive layers (as is usually the 

 case), they lose their characteristic shape and ap- 

 proach more and more the spherical outline as we 

 recede from the surface, until the deepest and there- 

 is 



