140 



FUNDAMENTALS OF STRUCTURE AND FUNCTION 



idL "bouiillus/ 



Anthro^ bacillus^ 



As to shapes, their name is legion. A typical cell might be thought 

 of as a spherical or ovoid body, but we find them cubical, flat, thread- 

 like, spindle-shaped, columnar, or irregular in outline. They are often 



modified by being com- 

 pressed by other cells, 

 but frequently if given 

 opportunity will resume 

 their original form when 

 released from pressure. 



Structural differences 

 exist between plant and 

 animal cells, the chief of 

 which is the cellulose wall, 

 characteristic of plants, 

 which gives such cells 

 the rigidity and yet the 

 flexibility found in woody 

 stems. Other physiolog- 

 ical differences will be 

 discussed in the following 

 chapters. 



red. corynxsc^s^ 

 .of f nog. 



a.no5om.<a 



r<Ed corp 

 of )inoa7 



Cugle: 



no. 



Spex-m ofTnarj. 



human 

 livei- cell 



Comparative size of cells. The anthrax bacillus 

 shown is among the largest of the bacteria, while 

 the human liver cell is not large as cells go. 

 (After Wells, Huxley, & Wells.) 



Why Cells Divide 



Every cell has its limits 

 of size and when that 

 size is reached, if food is 

 sufficient and conditions 

 favorable, it will divide. In both plant and animal cells, the mech- 

 anism and the. end results reached by cell division are similar, in 

 that the chromatin from within the nucleus is redistributed so that 

 the daughter cells have approximately the same amount of chromatin 

 and eventually the same size as the parent cell from which they 

 came. Cell division is a universal phenomenon and seems to be a 

 part of the normal life of cells. Theories advanced to account for 

 cell division are (1) colloidal changes in the protoplasm of which 

 they are composed, (2) electrical changes within the cell, (3) oxidative 

 changes within the cell, and (4) changes in surface tension. The 

 latter can be experimentally proven by treating unfertilized eggs with 

 certain chemicals which cause a change in surface tension and initiate 

 subsequent cell division. 



