parts: a more or less central mass, the nucleus 

 and a surrounding part, the cytoplasm. Both 

 of these parts of the protoplasm possess an 

 essentially similar chemical composition, ex- 

 cept that certain of the proteins, namely the 

 DNA-nucleoproteins (p. 90), are distinctive 

 of the nucleus. This is a very important dis- 

 tinction, and will be explored more fully 

 later (Chaps. 4, 8, and 27). 



Each cell, essentially, is a living unit. Some 

 small forms of life consist each of a single 

 cell, and larger plants and animals are com- 

 posed of many cells, variously modified ac- 

 cording to their special functions in different 

 parts of the body. Regardless of whether 

 they are unicellular or multicellular, how- 

 ever, each specifically distinctive kind of liv- 

 ing creature is designated as an organism. 



Recognition of the cell principle (Chap. 

 2) did not reach full maturity until late in 

 the nineteenth century. Nevertheless, this 

 principle has had tremendous impact upon 

 modern biology. Whatever activity is ex- 

 hibited by a multicellular organism in per- 

 forming the functions of responsiveness, 

 growth, and reproduction represents the sum 

 total of the activities of the component cells, 

 working together as a beautifully integrated 

 team. Accordingly, Part One of this book 

 (Chaps. 1 to 11) will deal with single cells. 

 This Part will attempt to analyze the struc- 

 ture and behavior of various basic cell types; 

 the remaining Parts will be concerned mainly 

 with the integrated behavior of the com- 

 ponent cells of higher plant and animal 

 organisms. 



In the present century, particularly during 

 the past 20 years, great advances have oc- 

 curred that have enriched our knowledge of 

 cellular structure and function. We now 

 begin to understand the significance of many 

 intracellular structures, or organelles. We 

 know quite a bit about how chromosomes 

 (p. 23) are constituted and how they are able 

 to transmit coded directions in each cell; 

 what the nucleolus (p. 23) is and how it is 

 concerned with the synthesis of specific pro- 

 teins in each cell; the nature of the mito- 



Life and Protoplasm - 7 



chondria (p. 28), ribosomes (p. 25), and the 

 endoplasmic reticulum (p. 23); and what 

 roles these structures play in physiological 

 activity. These and many other questions are 

 being probed intensively by research workers 

 throughout the world today. Final answers, 

 of course, cannot be given, but the current 

 status of the more important developments 

 will be discussed in several later chapters. 



All unequivocally living forms display a 

 cellular structure, but there are some border- 

 line cases, which must be mentioned now. 

 These include two groups of exceedingly 

 small infectious bodies — the Rickettsia (p. 

 580) and the viruses. Here, however, only the 

 viruses will be discussed. 



VIRUSES 



Viruses are disease-inducing particles that 

 are exceedingly small — so small in fact that 

 their structure cannot be revealed by any 

 type of light microscope (Fig. 2-10). Yet 

 viruses display some properties that else- 

 where are found only in association with liv- 

 ing cells. It may be necessary, therefore, to 

 qualify our concepts of the living unit, in 

 light of further knowledge. 



Ever since the dawn of biological curiosity, 

 when the early cave men first began to draw 

 pictures of the plants and animals that 

 shared their environment, man has continued 

 to discover and record new forms of life. 

 Before the seventeenth century, when the 

 microscope first revealed a whole new world 

 of living minutiae, generation after genera- 

 tion of protozoans, bacteria, and other micro- 

 scopic forms had lived and died without the 

 blessing of man's cognizance. The biologists 

 of that day were slow to admit these new 

 organisms into the fraternity of life, and 

 many years of research and controversy fol- 

 lowed before the microorganisms were recog- 

 nized generally. Today, it is the viruses that 

 seem to lie at the boundary line between the 

 living and the nonliving. If biologists finally 

 conclude that the viruses are alive, then it 

 will have to be admitted that a continuous 



