THE BIOLOGICAL UNIT 



39 



these microorganisms were immortal because they 

 never die, experiments have been made to de- 

 termine just how long some of them can go on 

 dividing. Professor Woodruff of Yale has been 

 carrying on an experiment during the past ten 

 years. During this time more than 5000 genera- 

 tions of descendants have been recorded from a 

 single animal (Paramecium). (Fig. 15.) In this 

 brief period he has been able to review many 

 more generations than would be possible in the 



Figure 15. This is a drawing of protozoa, Paramecium 

 aurelia, which Professor Woodworth has observed to divide 

 5000 times in a manner similar to that shown in Figure i2. 

 The surface of the body is covered with minute cilia which are 

 used in locomotion and in securing food. The large mass and 

 the two small black dots are nuclei. Scattered food vacuoles 

 occur in the cytoplasm as well as two excretory vacuoles, shown 

 as clear circles in the figure. 



more slowly reproducing forms of life. For such 

 animals as paramecium maintain their individuali- 

 ty for not more than 12 hours at the longest, under 

 normal conditions. Let us compare man with 

 this protozoan. We assume that three genera- 

 tions of men will be produced in each century. 



