44 
ZOOLOGY: L. L. WOODRUFF 
Proc. N. A. S. 
reorganization of the cell (endomixis) without the cooperation of two 
animals involving syncaryon formation (amphimixis) as in conjugation. 
After endomixis was worked out, it seemed unnecessary to continue 
longer the main pedigreed culture and accordingly on May 1, 1915, at the 
5071st generation, the experiment was formally considered as closed. 
The record of divisions for each year of the life of the culture to May 
1, 1915, may be tabulated as shown in the preceding table. 
Thus for eight years the culture averaged about fifty generations per 
month. However, at the formal termination of the experiments in 1915, 
I was reluctant to discard the race and was tempted to keep it under 
control but without such exacting daily observation and recording 
of the daily division rate. In this manner it has been maintained up to 
the present (December, 1920). But, from time to time, thirty-day tests 
have been made of the vitality of the race under the former rigid culture 
conditions, and in each case the same general average division rate has 
been revealed as during the first eight years of its life. For example, 
during December, 1915, 52 generations; during December, 1917, 56 genera- 
tions; and during November, 1920, 54 generations were attained. On the 
basis of such tests it is fair to make the exceedingly conservative estimate 
of 600 generations attained each year since May 1, 1915. This would 
give, in round numbers, 8400 generations attained by the culture during 
the 13V2 years of its life to date. 
Thus the conclusion is still justified that conjugation is not a necessary 
phenomenon in the life history of Paramecium aurelia under favorable 
environmental conditions. But there is an internal reorganization process 
(endomixis) which occurs periodically. Whether endomixis is a necessary 
factor for the continuance of the race is another question — a new question 
which has been raised by these studies, and is now under investigation. 5 
1 Woodruff, L. L., "The life cycle of Paramecium when subjected to a varied environ- 
ment," Amer. Naturalist, 42, 1908. "Two thousand generation of Paramecium," A rchiv 
f. Protistenkunde, 21, 1911. 
2 Woodruff, L. L., "So-called conjugating a non-conjugating races of Paramecium," 
J. Exper. Zool., 16, 1914. 
3 Woodruff, L. L. andBai-sell, G. A., "Rhythms in the reproductive activity of In- 
fusoria," J. Exper. Zool., 11, 1911; "The temperature coefficient of the rate of reproduc- 
tion of Paramecium aurelia," Amer. J. Physiol., 29, 1911. 
4 Woodruff, L. L., and Brdmann, R., "A normal periodic reorganization process with- 
out cell fusion in Paramecium," J. Exper. Zool., 17, 1914. 
5 Woodruff, L. "The problem of rejuvenescence in protozoa," Biochem. Bull., 4, 
1915; "The influence of general environmental conditions on the periodicity of endomixis 
in Paramecium aurelia," Biol. Bull., 33, 1917. 
