578 



CONTINUITY OF LIFE 



We might start by considering how 

 growth occurs in single cells, Protozoa for 

 example. If one or only a few Protozoa are 

 placed in a flask containing complete nu- 

 trients for this particular cell, in a short 

 time there will be millions. Since they are 

 single cells, we can count them at regular 

 intervals during this period of increase and 

 from this information construct a curve 

 which will tell us something about how 

 growth occurs (Fig. 23-16). Such a curve is 



total number remains the same. This is 

 called the stationary phase. This curve can 

 be repeated again and again by simply tak- 

 ing some of the organisms in the stationary 

 phase and placing them in a fresh medium. 

 If the medium is continually changed, the 

 culture can be kept in the logarithmic 

 phase indefinitely. Apparently, these cells 

 can continue to grow and divide indefinitely 

 at a uniform rate with no sign of aging. 

 Aging then may be only the exliaustion of 



number of - 

 protozoa 



time 



Fig. 23-16. Growth of cells, such as Protozoa in a flasi< of nutrients, follows very precise stages. 

 During the initial stage there is little increase in numbers (lag phase). This is followed by a 

 rapid uniform rate of increase (logarithmic phase) that continues until the food becomes 

 exhausted or the accumulated wastes become toxic, or both factors operate simultaneously. 

 Growth then remains at a plateau for a time (stationary phase). This cycle can be repeated 

 any number of times by transplanting some of the cells to a fresh medium. 



said to be sigmoid, because it is S-shaped. 

 At the beginning the cells fail to divide for 

 a time, as indicated on the curve by the so- 

 called lag phase; just why this occurs is not 

 known. They then begin to divide at a 

 rapid and uniform rate. This is called the 

 logarithmic phase of growth because the 

 cells increase in a geometric manner, that 

 is, 2, 4, 8, 16, etc. Once they reach this 

 phase they continue dividing at a uniform 

 rate, so the line is straight. As the limits of 

 food in the culture are reached or the ac- 

 cumulation of wastes inhibit further divi- 

 sions, the population gradually falls off until 

 it finally reaches a plateau. Either the or- 

 ganisms no longer divide, or if they do, they 

 die as fast as they are produced, for the 



food or the accumulation of wastes in any 

 community of cells. 



The Protozoa in this culture are all alike 

 and are not complicated by differentiation, 

 as would be the case in a growing embryo. 

 Can this information be applied to embry- 

 onic cells? Tissue cultiue studies give us 

 some information on this point. By remov- 

 ing a small bit of embryonic chick heart, for 

 example, and placing it in a flask containing 

 adequate nutrients, growth of the cells will 

 occur following the same curve as that dem- 

 onstrated for the Protozoa. At first the cells 

 divide very slowly, then increase to a uni- 

 form rate, and finally fall off until no more 

 divisions occur. If a small bit is transferred 

 to a fresh flask of medium, the cycle will be 



