AGAMIC REPRODUCTION AND REJUVENESCENCE 137 



during the last few years that the occurrence of conjugation 

 is dependent, at least in a large measure, upon external factors. 

 Woodruff has experimentally induced conjugation in members of 

 his culture which has been agamically bred through thousands of 

 generations. Jennings concludes from extended experimentation 

 that conjugation does not bring about rejuvenescence, but merely 

 increases variability, while Calkins and Gregory believe that reju- 

 venescence does occur, at least in some cases. 



If conjugation is not a necessary feature of the life cycle, or if 

 it fails to accomplish rejuvenescence, two alternative conclusions 

 present themselves : either these animals do not necessarily undergo 

 senescence or else rejuvenescence is accomplished in some other 

 way than by conjugation. The relation found to exist between 

 agamic reproduction and rejuvenescence in the flatworms suggests 

 at once the possibility that a similar relation may exist in the 

 protozoa. 



Since the protozoa are unicellular animals, agamic reproduction 

 is essentially a process of cell division, but since it is also true that 

 at least many protozoa possess a more or less complex morphological 

 structure, agamic reproduction, as in multicellular forms, resembles 

 the process of reconstitution in that it involves various morpho- 

 logical changes, consisting in the dedifferentiation and disappear- 

 ance of certain structures and the formation and development of 

 others. In Paramecium, for example, agamic reproduction does 

 not consist merely in nuclear and cytoplasmic division, but exten- 

 sive reorganization also occurs. In Figs. 41-43 the most important 

 changes are diagrammatically represented. Fig. 41 shows the 

 animal before division, the oral groove, og, the pharynx, p, and the 

 two vacuoles, v, being indicated in the figure, as well as the meganu- 

 cleus, mgj and the micronucleus, me. The first indications of 

 division are cytoplasmic, not nuclear, and consist in the formation 

 of a new contractile vacuole in what is to become the anterior region 

 of each individual, the two vacuoles of the parent individual becom- 

 ing the posterior vacuoles in the daughter animals and new vacuoles, 

 v'v', appearing in the anterior region of each. The mouth and 

 pharynx and the posterior portion of the oral groove undergo more 

 or less change and become parts of the posterior daughter animal, 



