CELLS AND TISSUES 39 



derm cells of coelenterates and flatworms, have food vacuoles in which 

 food is digested. Digestive enzymes are secreted from the cytoplasm into 

 the cavity of the vacuole, the food particles are digested and the products 

 of digestion are absorbed through the vacuolar membrane into the cyto- 

 plasm. The protozoa living in fresh water have the problem of eliminat- 

 ing the water which enters the cell constantly by osmosis (p. 51). These 

 forms have evolved contractile vacuoles, which alternately fill with water 

 from the adjacent cytoplasm and then eject the water to the surrounding 

 environment. 



Most animal cells are quite small, too small to be seen with the 

 naked eye. The diameter of the human red blood cell is about 7.5 

 microns (a micron is 0.001 millimeter), but most animal cells have 

 diameters ranging from 10 to 50 microns. There are a few species of 

 giant amebas with cells about 1 mm. in diameter. The largest cells are 

 the yolk-filled eggs of birds and sharks. The egg cell of a large bird such 

 as a turkey or goose may be several centimeters across. Only the yolk 

 of a bird's egg is the true egg cell; the egg white and shell are noncellu- 

 lar material secreted by the bird's oviduct as the egg passes through it. 



The limit of the size of a cell is set by the physical fact that as a 

 sphere gets larger, its surface increases as the square of the radius but its 

 volume increases as the cube of the radius. The metabolic activities of 

 the cell are roughly proportional to cell volume. These activities require 

 nutrients and oxygen, and release carbon dioxide and other wastes which 

 must enter and leave the cell through its surface. The upper limit of 

 cell size is reached when the surface area can no longer provide for the 

 entrance of enough raw materials and the exit of enough waste products 

 for cell metabolism to proceed normally. When this limit is reached the 

 cell must either stop growing or divide. 



11. Mitosis 



Because of the limitation on the size of individual cells, growth- 

 the increase in protoplasmic mass-is accomplished largely by an increase 

 in the number of cells. When a single-celled protozoan divides, the 

 resulting two cells are separate individuals, members of a new generation. 

 In multicellular animals, cell division results in an increase in the num- 

 ber of cells per individual, but the process of cell division is funda- 

 mentally the same in both. This process of cell division, called mitosis, 

 is extremely regular and ensures the qualitatively and quantitatively 

 equal distribution of the hereditary factors between the two resulting 

 daughter cells. Mitotic divisions occur during embryonic development 

 and growth, in the replacement of cells that w^ear out, such as blood cells, 

 skin, the intestinal lining, and so on, and in the repair of injuries. 



When a dividing cell is stained and examined under the microscope, 

 dark-staining bodies, called chromosomes, are visible within the nucleus. 

 Each consists of a central thread, the chromonema, along which lie the 

 chromomeres-small, beadlike, dark-staining swellings. In a cell which 

 is not dividing, chromosomes are usually not visible as separate entities; 

 instead the nucleus contains an irregular network of fine chromatm 



