244 EMBRYOLOGY 



sponsible for this self-duplication, and these substances must be synthesized 

 during growth. 



Growth in relation to time 



We shall begin the analysis by first considering the over-all growth 

 process in an organism. The generalized pattern of growth of an organism 

 with respect to time follows an S-shaped curve. The weight, or size, or any 

 mass unit is plotted against any time unit — hours, minutes, or whatever the 

 case may be. The organism starts with a definite size, increases very slowly 

 for a while, and then begins to grow very rapidly (Fig. 6). For a period the 

 increase may be at a rather constant rate and the slope of the curve does not 

 change. But soon certain factors cause a slowing up of growth, a falling off 

 of the increase in size with time. And finally no further increase in size of the 

 organism takes place. When this stage is reached an equilibrium is established 

 between dying cells, which would tend to reduce the weight of the organism, 

 and new cells resulting from division, which would tend to increase the 

 weight. Thus, in an adult with a stable weight cellular growth continues in 

 many organs of the body, but because of loss or death of cells, no over-all 

 growth results. This gradual decrease in over-all growth is one aspect of an 

 aging process. 



Since the actual growth curves of some organisms show considerable 

 deviation from the S shape, we shall attempt to outline some of the factors 

 influencing growth. 



Growth in microorganisms 



To analyze the over-all growth curve a little further, we shall discuss some 

 studies on such microorganisms as protozoa and yeast, in which we are able 

 to count the number of cells at given intervals and follow growth in this 

 manner. Let us consider, first of all, the growth of microorganisms in a 

 limited volume of nutrient medium. A culture flask of medium is seeded with 

 a few protozoa or yeast and the number of individuals is measured as time 

 progresses. The individual cells begin to divide rather slowly at first; later 

 they reach a very constant rate of division with the number of individuals 

 doubling at regular intervals (Fig. 162). This phase of growth is termed the 

 logarithmic phase, and in the curves plotted the logarithm of the number of 



