THE VITAL UNITS CALLED CELLS 1 97 



to mention here. This much, however, we do know: that 

 the cell itself is a complete and indivisible unit. Attempts 

 to distinguish hving and non-living elements in it are futile 

 and irrational. 



MULTICELLULAR AND UNICELLULAR ORGANISMS CONTRASTED 



Thus far we have stressed one of the principal tenets of 

 the cell theory, which is indeed an estabhshed fact, namely, 

 that the body is a sort of kingdom of cellular units. This 

 apphes not only to man but to almost all living animals 

 and plants. The word almost is inserted because forms of 

 hfe exist which are themselves single cells, not combinations 

 of cells. We at once think of the bacteria and of certain 

 unicellular animals, the Protozoa. As an example of the 

 latter, the parasite of malaria is cited. There is a distinct 

 difference between the life of a unicellular organism and a 

 cell inhabiting our own body. 



Perhaps this may be made clear by reference to Figure 3. 

 A unicellular organism like an ameba (a) has 'to adjust 

 itself only in respect to its own environment (e). A cell 

 of the intestine (i), on the other hand, must shape its 

 behavior not only in response to the character of the contents 

 of the intestine (e) but also in respect to neighboring cells 

 (c) and the fluids of the body (f). In the case of epithelial 

 cells of the skin the contact with adjacent cells is often 

 not merely the close approximation of like surfaces. There 

 may be continuity of living substance across specially 

 developed bridges which pass from one cell to another. 

 Cellular activity may also be governed by nerve fibers 

 terminating on their surfaces so that stimuli originating 

 in other parts of the body impinge upon them. The associa- 

 tion with the body fluids is a complex matter involving the 

 transport of substances of many kinds to and from the cell. 

 It is interesting to note that we have among these the 

 so-called chemical messengers, or hormones, produced by the 

 ductless glands, and probably by other cells not recognized 

 as glandular in nature. Through their action one cell may 

 influence another far removed from it. A free living pro- 

 tozoan or a bacterium is, therefore, not strictly homologous ^GilC/O 



OS 



