110 PROTOPLASMIC AGE OF PROTOZOA 



the source of such food material. Upon such a continually changed 

 diet he carried on a race of Paramecium aurelia through several hun- 

 dred generations without the advent of a period of depression. It 

 appears, therefore, that in the constantly changing conditions of 

 nature a race of protozoa may live much longer than under the 

 conditions of laboratory experiments on a single diet. It is probable 

 that the salt contents of the medium rather than the food are of 

 importance in this connection, since the bacteria of the laboratory 

 air, with which all food media were inoculated, were presumably 

 the same. 



II. MORE COMPLICATED LIFE CYCLES AND THE PERIODS OF 

 "YOUTH," "MATURITY," AND "AGE." 



With different types of protozoa the three periods of vitality may be 

 recognized with quite the same facility as in any of the lower forms of 

 metazoa. There is no sharply defined difference between them, but, 

 as Maupas first pointed out, there is a fairly definite period of proto- 

 plasmic or " individual" maturity, which is preceded by a period that 

 may be designated "youth," and is followed by a period that may be 

 called "old age." The period of maturity is so frequently accom- 

 panied by well-marked cellular changes, which distinguish the organ- 

 isms at that period from the ancestral cells which gave rise to them, 

 that we are justified in the attempt to generalize, if only for descriptive 

 purposes, and to speak of periods of youth, maturity, and age in 

 protozoa. 



In the life history of Paramecium aurelia the three periods, youth, 

 maturity, and age, of the life cycle are not so clearly marked by struc- 

 tural and functional manifestations as in some other forms of protozoa. 

 Nevertheless, there is a physiological difference which becomes appar- 

 ent when one follows out the complete history. The period of youth 

 is marked by a high rate of division energy and by the fact that con- 

 jugation does not occur if many of them are put together in a limited 

 space. After some time in culture, however, usually when the rate of 

 division has begun to decline, the protoplasm of the cell body 

 changes slightly in physical and chemical make-up, so that two or more 

 cells upon meeting fuse and conjugate. The entire race of Para- 

 mecium in such a culture may become sexually mature at the same 

 time, and "epidemics" of conjugations may be thus obtained. At the 

 last period of depression, however, in the experiments cited, there were 

 no conjugations, a fact indicating, possibly, the exhaustion of the germ 

 plasm. Such a final period of old age may be easily identified, involv- 

 ing, as it does, the curious vacuolization and degeneration of the 

 protoplasm and exhaustion of the physiological energies. 



