72 ORGANISMS OF ONE CELL 



the continued division of a single original egg cell. The 

 entire race of Paramecium derived from a single ancestral cell, 

 and not any single cell of that race should therefore be compared 

 with a metazoon, the race of Paramecium in most cases may 

 die from old age no less surely than the race of cells composing 

 the metazoon. Old age and natural death, therefore, appear 

 to be 'characteristic of animals whether single cells or many 

 celled. 



In spite of the fact that protozoa, should they escape the thou- 

 sands of their natural enemies, may die of old age, they never- 

 theless exist in more or less abundance in natural waters, and 

 will undoubtedly continue to exist in the future. The question 

 then arises, how is this physiological weakness overcome and 

 what means are employed in nature to perpetuate the species? 

 One means of accomplishing this end is the chemical reorgani- 

 zation following endomixis as described above; another and 

 an essentially similar means is the chemical reorganization of 

 the protoplasm which follows fertilization through the act of 

 conjugation. Biitschli in 1876 observed that a culture of Para- 

 mecium, after some weeks in a watch glass, showed hundreds 

 of pairs in conjugation, the cells being united in the region of 

 the peristomial area. This union or conjugation, lasting from 

 eighteen to twenty-four hours, is followed by separation of the 

 two individuals (Fig. 29). From this observation Biitschli 

 concluded that conjugation is for the purpose of renewing the 

 vitality of the race, or is a means of protoplasmic rejuvenes- 

 cence, and this interpretation, while it has been questioned, 

 has never been disproved or improved. 



Conjugation or Fertilization. Conjugation of protozoa is 

 essentially the same as fertilization in metazoa, and in one form 

 or another represents a phenomenon practically universal in 

 animals and plants. It is, therefore, one of the fundamental 

 activities of living things (see page 15). It is usually associated 

 with reproduction, but reproduction may go on without it, as 

 in the case of division, spore formation, etc., of the protozoa, 

 or budding in Hydra and plants, or parthenogenesis in insects. 

 Strictly speaking, therefore, it is not a process of reproduc- 



