^Protoplasm, the Cell, 

 and the Organism 



UNTIL about 125 years ago, very little was 

 known about cells. Since then, however, bi- 

 ologists have come to realize that the living 

 parts of all definitely living things are com- 

 posed of cells — one or a few in smaller 

 organisms and many millions in larger forms 

 such as man (Figs. 2-1 and 2-2). This cell 

 principle (p. 38) has had tremendous influ- 

 ence upon all phases oi biology. Accordingly, 

 the basic organization of this text centers 

 around the form and function of cells, taken 

 individually ami in highly organized groups. 



THE CELL 



The cell ma) - be defined as an organized 

 unit mass of protoplasm, consisting of two 

 complementary, mutually dependent parts: 

 a more or less central part, the nucleus, and a 

 surrounding part, the cytoplasm. Generally 

 the nucleus is delimited from the surround- 

 ing cytoplasm by an exceedingly delicate 

 nuclear membrane (Fig. 2-2) and the cyto- 

 plasm is bounded externally by a specialized 



16 



layer, also very thin and delicate, which is 

 called the plasma membrane. However, in 

 addition to these intrinsic membranes, which 

 are specialized parts of the protoplasm 

 proper, most cells are covered by a layer of 

 relatively inert material, which stabilizes the 

 protoplasmic surface. The composition of 

 such extrinsic, or nonprotoplasmic cell cov- 

 erings varies according to cell type (p. 33). 

 But regardless of type, the boundary mem- 

 brane oi a cell, consisting of the plasma mem- 

 brane plus such extrinsic material as may be 

 present, is often referred to as the cell mem- 

 brane. 



Some small primitive unicellular organ- 

 isms, especially the blue-green algae (p. 594), 

 do not seem to show a distinct segregation of 

 nuclear and cytoplasmic materials (Fig. 2-3). 

 In such cells, the DNA proteins — a kind of 

 material that in other cells is confined within 

 the nucleus — appears to be scattered through- 

 out the cytoplasm in the form of numerous 

 fine granules, called chromidia. In other 

 words, these cells contain nuclear material. 



