Thirteen Thousand Generations of Paramecium.

Abstract

As a matter of record it seems well to summarize the data and give the present status of the long-continued culture of Paramecium aurelia at Yale University.

Can unicellular animals reproduce indefinitely without recourse to fertilization? This was the question to which an answer was sought when this experiment was begun 22 years ago. On May 1st, 1907, a pedigree culture of Paramecium aurelia was started with a “wild” individual and has been maintained by the isolation of certain of its progeny practically every day, with the exception noted below. The number of divisions has been recorded at each isolation and has afforded a continuous series of data for the study of the reproductive activity of the culture. Throughout the experiment some of the animals discarded from the lines at the daily isolations have been fixed, stained, and mounted as permanent preparations for the study of the cytological changes during the life history. The culture medium for the first 8 months of the work consisted of infusions of hay and fresh grass, but since February, 1908, various materials collected from ponds, swamps, etc., have been employed. The infusions have invariably been thoroughly boiled to prevent the contamination of the pure culture with foreign strains of Paramecium. The possibility of conjugation occurring in the culture has been precluded by the almost daily isolation of the products of division. In short, the animals of the culture are all direct lineal descendants, without fertilization, of the single animal isolated in 1907.

In this manner the pedigree culture was carried for 8 years, during which 5071 generations were attained, and then (May 1, 1915), the experiment was considered formally closed with the statement that the organisms of the present generation are in as normal morphological and physiological condition as the original individual isolated to initiate the culture.