OUR APPROACH 9 



nutrition, and reproduction (See Chapters VIII and IX), all 

 living things manifesting these activities in one way or another. 

 In the so-called metazoa (many-celled animals) certain parts 

 of the body, the organs (such as heart, stomach, kidneys, etc.) 

 are specialized to perform certain activities, and the "higher" 

 the type of organism the greater is the specialization. Con- 

 versely, organisms of "lower" types have fewer organs and each 

 organ does proportionately more varied physiological work. 



All Living Things Composed of Cells 



In 1838 to 1839 a great step in the advancement of biological 

 knowledge was taken by Theodor Schwann, a German anatomist, 



# *v 



Fig. 2— CELLS FORMING A 

 TISSUE 



Each cell has its own walls, nucleus, 

 and cytoplasm or cell body which 

 are formed by division of pre-existing 

 cells, a, a cell with its nucleus in 

 an early stage of division; b, a nu-/ 

 cleus in the midphase of division; 

 c, two young cells just after division. 



From a photomicrograph by the author 

 Magnification, 600 



who, with the co-operation of Matthias Jakob Schleiden, a bota- 

 nist, formulated the Cell Theory of Organic Structure * This 

 is now a basic fact in biological science. All living things are 

 made up of units of structure called cells (Fig. 2). Thus, a 

 higher type of organism is not merely a larger mass of living 

 protoplasm, but it is also composed of numerous minute units, 

 or cells. The cells contain living protoplasm, through the activity 

 of which the supporting structures found in all higher animals — 

 cartilage, bone, shells, skeletons, wood, etc. — are formed. The 

 cells are specialized to perform various functions and activities 

 and are arranged in sheets, or groups, of like kinds, so that we 

 * See "The Plant World" in this Series, pages 4-6. 



