246 EMBRYOLOGY 



cells is represented along the ordinate and time along the abscissa. A straight 

 line thus means that the number of animals is doubling at regular time in- 

 tervals. After a period during which the rate of cell division remains constant, 

 the rate declines and finally the number of individuals in the culture does not 

 undergo any further increase. 



If some of the organisms which have stopped dividing are removed from 

 this culture medium and placed in a fresh culture medium, they go through 

 this same cycle. Growth starts out very slowly, then begins to reach a constant 

 rate, and falls off again (Fig. 162). Thus the growth of protozoa and yeast, 

 goes through a specific cycle of slow growth, rapid growth, decreasing 

 growth, and, finally, a stationary state. This is a type of cellular growth 

 which is not complicated by differentiation of cells into various cell types as 

 occurs during the growth of an embryo. 



Factors which modify growth 



A number of factors influence the extent to which growth will take place 

 in a culture medium. If the concentration of food in the culture medium is 

 increased, the period of time during which the animals divide is prolonged. 

 Thus the straight-line portion of the curve will continue for a longer period, 

 but after a time it will level off again (Fig. 162) . The increase in population 

 is due to an increased concentration of food. If the concentration of food is 

 increased a second time, the curve will rise to a greater extent but again it will 

 level off. Thus one limiting factor is food. But in a limited volume it is not 

 possible to continue to force growth merely by increasing the food. Even with 

 increased concentration of food the animals will still stop dividing. Another 

 factor is aeration. Shaking the culture medium or bubbling oxygen through 

 it increases the logarithmic phase of growth and a larger population of cells 

 results. A culture in a flask which is not aerated will reach its maximum popu- 

 lation level sooner than one which is aerated. Even an aerated culture with 

 the highest possible food concentration, however, will still reach a maximum 

 population, and growth will cease. This limitation of growth always takes 

 place in a limited volume. 



If the volume is unlimited, as it is to some extent when organisms are 

 grown in a long tube, a different result is obtained. Visualize a long tube 

 partly filled with a nutrient medium; such a tube may be inoculated at one 

 end with a microorganism such as a mold. Under these conditions the mold 



