38 - The Cell 



the body in definite channels. Likewise, the 

 nervous system effects an integrated behavior 

 of the body parts in higher animals generally. 

 These connections enable the cells, tissues, 

 and organs of the multicellular body to act 

 together as a unified whole — the organism. 

 But whatever the organism does is done by 

 its cells, and the noncellular parts of an or- 

 ganism are lifeless products and passive tools 

 subserving the activities of the living cells. 

 Thus the life of the organism is to be con- 

 sidered as the sum total of the integrated ac- 

 tivities of its component cells. To consider 

 that the organism is "something more than 

 the sum of its cells" is like considering that a 

 machine is something more than the total of 

 its parts. Without question, the properties of 

 the whole machine are determined by the 

 relations, as well as by the structures, of the 

 separate parts — and how the parts are fitted 

 and linked together is just as important as 

 how each is formed individually. Likewise 

 the activities of an organism result from the 

 interaction, rather than from the inde- 

 pendent action, of the component cells, 

 organs, and organ systems. 



The tissue cells of a complex animal sub- 

 ordinate themselves to the functions of the 

 whole organism, but all the cells are indi- 

 vidually alive, and some retain an unlimited 

 potential for growth and multiplication. 

 This can be shown by the technique of tis- 

 sue culture. In this technique a small piece, 

 usually of embryonic tissue, is placed in a 

 nutrient medium, such as the blood plasma 

 of the same animal. Optimum conditions are 

 provided for the cells by changing the 

 medium at frequent intervals, meanwhile 

 trimming away excess quantities of tissue as 

 growth continues. The utmost care to pre- 

 vent bacterial contamination must be taken 

 during all these operations. 



In tissue culture, some cells retain their 

 characteristic form and activity and their 

 capacity to grow quite indefinitely. At the 

 Rockefeller Institute, for example, a piece 

 of heart tissue, taken from a chick embryo, 

 was cultured for more than 20 years. In this 



tissue some of the cells remained motile and 

 continued to grow throughout the whole 

 period. In fact, if all this growing tissue could 

 have been saved and provided with good con- 

 ditions, the accrued amount of new tissue 

 would now be many times the volume of 

 this earth. 



THE CELL PRINCIPLE 



The cell principle has gradually developed 

 from the cell hypothesis, and now it has at- 

 tained a very fundamental importance in 

 biology. This development may be sum- 

 marized as follows: 



J. Plant and animal bodies, with minor 

 exceptions, are composed of one or more 

 cells. Among the first to emphasize this 

 generalization were a botanist, Mathias 

 Schleiden (1838), and a zoologist, Theodor 

 Schwann (1839), who submitted the cell hy- 

 pothesis almost simultaneously. Much earlier 

 Robert Hooke (1665) had observed plant 

 cells, and Anton Leeuwenhoek (1660) had 

 observed animal cells. However, these early 

 investigators could not have realized the 

 general occurrence and wide significance of 

 the cell. 



2. The essential living part of everv cell 

 is its protoplasm. In the early development 

 of the cell hypothesis, the cell wall (or the 

 pellicle) was thought to be more important 

 than the protoplasm; and it was not until 

 1861 that Max Schultze recognized that the 

 protoplasm alone displays the essential attri- 

 butes of life, in both plant and animal cells. 



3. New cells arise only by division from 

 pre-existing parent cells — at least under con- 

 ditions as they exist today. Accordingly each 

 species of plant and animal represents an 

 unbroken cell lineage extending back into 

 ancient time. The first of these conclusions 

 was stated clearly by Virchow in 1855; the 

 second derives from the work of August Weis- 

 mann in the latter part of the nineteenth 

 century. 



■/. Each cell — so long as it retains its ca- 

 pacity lor growth and multiplication — must 



