108 PROTOPLASMIC AGE OF PROTOZOA 



This exhaustion of the power to digest and assimilate is an unmis- 

 takable phenomenon in the life history of a protozoon, and marks a 

 somewhat indefinite phase of the life history, which was designated 

 the "period of depression." Many other observers have noted it in 

 connection with protozoa of different kinds; the first, Biitschli, in 1876, 

 in relation to paramecium, without noting the sequence of stages lead- 

 ing to this depression period, observed that the organisms become 

 reduced in size and sluggish in movement, and that while in such 

 condition they conjugate, an observation which led him to his famous 

 suggestion that conjugation is not an act of reproduction, but a means 

 of renewing the vitality of the organisms, including the power to repro- 

 duce ; in other words, a Verjungung of the protoplasm. Later observers, 

 including Maupas and Hertwig, likewise studying the organisms en 

 masse, noted a similar stage of lowered vitality, the former concluding 

 that it indicates a senile degeneration of the nuclei, the latter, that it 

 indicates a changed relation between the volume of the nucleus and 

 that of the cell. Woodruff and Gregory, as graduate students in the 

 Columbia laboratory, have followed out, generation by generation, 

 the life history of different protozoa, the former in connection with 

 Oxytricha fallax, one of the hypotrichous infusoria, which he followed 

 for 860 generations of cell divisions, requiring twenty-one months, 

 the latter in connection with TilUna magna, one of the holotrichous 

 infusoria, which was followed for thirteen months, dying out in the 

 548th generation. Periods of depression were observed in these 

 organisms as in paramecium, and the same physiological derange- 

 ments were noted by both observers, the first period of depression 

 carrying off all the cells of tillina. 



What is the explanation of the depression period? The organisms 

 have abundant food; they are able to take in food up to a certain 

 time, but they appear abnormal in structure, and if left to themselves 

 they would die. The protoplasm at this period is markedly different 

 from that at other times; in paramecium the endoplasm lacks the 

 characteristic vacuoles of the ordinary organism and appears dense 

 and homogeneous (Fig. 39), an appearance due to the aggregation of 

 granules. The lack of vacuoles signifies a concentration of the cell 

 protoplasm and, therefore, a reduction in size of the organism; the 

 macronucleus, in the meantime, retains its full size, and it thus appears 

 that the volume of the latter is relatively greater than it is under normal 

 conditions. This is perhaps one reason why Hertwig, Popoff, and 

 others have concluded that the cause of depression is the change in 

 relative volume of nucleus and cytoplasm, but such a change in relative 

 volumes may be equally well an effect of the depression and not its 

 cause. Woodruff noted the same reduction in size of the cell in 

 oxytricha (his figures 1 and 9) during the period of depression and a 

 corresponding change in nature of the cytoplasm, which, in oxytricha, 



