132 PROTOPLASMIC AGE OF PROTOZOA 



protoplasm is now extended nearly throughout the cell, the extremities 

 alone retaining the granular structure of the depressed condition. 



After such successful stimulation the digestive processes recom- 

 mence, the organisms divide, and the division rate, as indicated by the 

 curve, rises to an average of more than one division per day (see 

 Fig. 38). 



Three times in the history of this paramecium culture were the cells 

 stimulated to new activity by this artificial means. The first time, 

 as stated, was after the 200th generation, the stimulant being beef 

 extract; the second time was after 198 generations more (398th of the 

 race), the stimulant being beef extract and potassium phosphate; the 

 third was after about 193 generations more (about the GOOth of the 

 race). This third period of depression was most interesting, for it was 

 found that the same stimulants that had been previously used with 

 success were now without effect; beef and potassium salts of various 

 kinds were tried in vain, and the final extinction of the race was threat- 

 ened; indeed, one race, which was called the B series, died out entirely 

 in the 540th generation. Only six cells were left, finally, for experi- 

 mentation, but some of these were successfully stimulated by treatment 

 with an extract of pancreas, which contains many different salts in 

 solution. The effect of this last stimulation was a renewal of the 

 vitality, but the potential given to the protoplasm was not so great nor 

 so clearly defined as in the previous periods of depression, and after 

 another six months, in which the organisms showed great sluggishness, 

 the race died in the 742d generation. This fourth cycle is the most 

 important for our present purpose, since it represents the period of old 

 age in the protoplasm under observation. The cells divided only 123 

 times, and toward the end manifested curious and hitherto unobserved 

 degenerative phenomena, which deserve special attention. 



The protoplasm of the cells in this final period of depression had 

 at first the same appearance as the protoplasm of the organisms at 

 previous periods of exhaustion; the cell body became granular, the 

 size decreased, and the general appearance was similar to that which 

 had been successfully met at previous periods. The same stimulants 

 were used; the diet was changed for short periods as before; and, 

 singularly enough, the same effect on the structures of the cell was 

 produced. The granules disappeared, the nucleus and cytoplasm 

 appeared perfectly normal, and the organisms were able to take in 

 food, digest, and assimilate it. The normal size was restored, and it 

 seemed, from morphological grounds, that the depression period had 

 been successfully overcome for a fourth time. Still, the cell divisions 

 were very infrequent and irregular, while the few that did take place 

 were mostly of a pathological nature, complete fission not taking place, 

 the result being monsters of different size and form (Fig. 55). The 

 macronucleus was perfectly normal in the last cells of the race, but the 



