70 ORGANISMS OF ONE CELL 



tions and without depression. In this race, however, a process 

 of parthenogenesis equivalent to conjugation, has periodically 

 reorganized the protoplasm. In essence, therefore, it appears 

 that the unicellular organisms agree with the multicellular in 

 possessing physiological powers which gradually wear out. To 

 be sure the single cells of the majority of generations do not 

 die. They cease to live as the same cells, but the protoplasm 

 continues to live in the daughter cells arising from divisions; 

 but the same is true of any individual cell of a metazoon since 

 the adult organism is formed by 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, and 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 ani- 

 mals whether single cells or many celled, although conflicting 

 results in the hands of different experimenters leave this 

 conclusion somewhat weakened. 



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

 sands of their natural enemies may die of old age they neverthe- 

 less 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? 

 Blitschli in 1876 observed that a culture of Paramecium after 

 some weeks in a watch glass showed hundreds of pairs in conju- 

 gation, 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 individ- 

 uals (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 rejuvenescence, and this in- 

 terpretation, while it has been questioned, has never been dis- 

 proved or improved. 



Conjugation or Fertilization. Conjugation of protozoa is 

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



