248 BIOLOGY OF THE PROTOZOA 



I. ISOLATION CULTURES. 



The study of protozoon protoplasm by the isolation culture 

 methods has thrown considerable light on these problems of general 

 vitality. If a bit of such protoplasm in the form of a single indi- 

 vidual organism, and its progeny by division, is maintained under 

 conditions of food and temperature as constant and uniform as 

 possible, then variations in vitality may be measured and compared 

 in relation to phenomena in the life cycle which are suspected of 

 playing a role in connection with the lability of that protoplasm. 



In order to study protoplasm in this manner it is necessary to 

 adopt some measure of vitality which will be an expression of the 

 sum-total of all vital activities. Since every function is a link in 

 the chain of vital activities any one function would do were it 

 possible to measure it accurately, but the difficulty comes with the 

 inability to measure excretion, or nutrition or irritability in any 

 complete and definite manner. Reproduction, however, can be 

 readily measured and being dependent upon the general functions 

 of metaJbolism, becomes an excellent measure of vitality in a relative 

 and comparative sense. In one way or another the division-rate 

 has been used^as a measure of vitality ever since Maupas, in 1888, 

 first attacked the problem of age and natural death in Protozoa by 

 the isolation culture method. 



In practically any free-living form of Protozoa if proper condi- 

 tions of food and temperature are provided, the general vitality or 

 sum-total of functional activity as measured by the division-rate, 

 continues more or less uniformly for long periods. The single 

 individuals thus watched appear to be self-sufficient and able to 

 continue their vital activities indefinitely. The question may be 

 raised as it has been raised repeatedly, does the protoplasm of such 

 an individual retain this constant potential of vitality indefinitely, 

 or like a machine, does it wear out sooner or later, and will it ulti- 

 mately stop altogether? 



The problem thus worded is only a partial restatement of the old 

 problem concerning life and death of unicellular organisms which 

 Weismann raised more than fifty years ago. He took the ground 

 that Protozoa do not grow old and do not die a natural death, both 

 of which are prevented by an individual dividing into two while in 

 full vigor. The two young ones thus formed by division leave no 

 parental corpse but share the old protoplasm between them and 

 they in turn grow and similarly divide, so that old age is impossible 

 and natural death inconceivable. Weismann further maintained 

 that these fateful phenomena— age and death are penalties which 

 the Metazoa must pay for their privilege of specialization and dif- 

 ferentiation into somatic and germinal protoplasm. Protozoa he 

 compared with the germinal protoplasm of Metazoa in common 



