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ZOOLOGY: G. N. CALKINS 
single individual cell, which have been maintained under identical conditions 
and fed daily with the same standardized culture medium. 
The usual method of isolating single individuals each day in fresh culture 
medium, has been used throughout; five "lines" of individuals of the same 
ancestry forming a 'series.' Being isolated each day, conjugation never 
occurs in the culture dishes. 
After the daily isolations, the residual individuals are either thrown away, 
or are put together in larger culture dishes containing fresh culture medium. 
Here they are kept for a period of two weeks without renewal of the culture 
medium. At first there is an abundance of food and the organisms multiply 
by fission until three or four thousand are present. Later, the food becomes 
exhausted, and, as is well known in some other cases, the transition from 
rich feeding to starvation induces conjugation, provided the organisms are 
sexually mature. This procedure constitutes a "conjugation test" which is 
carried out at weekly intervals. 
The number of divisions per day in each of the five lines of a series is re- 
corded; also the sum of the daily divisions, giving a total number of gen- 
erations to date in each line. The daily records of all lines of a series may 
then be averaged for successive periods of ten days each, such averages giving 
a convenient and accurate representation of the relative metabolic activity at 
different periods of the life cycle. 
From time to time, individuals that have undergone conjugation in the 
weekly conjugation tests, may be isolated to form the beginnings of filial 
series which are maintained in isolation cultures exactly as in the original 
series. From the methods employed in these tests, it is evident that such 
conjugations take place between rather closely-related individuals which are 
of the same age. The protoplasm affected by such conjugation, also, has 
had the same history and the same daily treatment throughout, as that main- 
tained in the isolation cultures. After conjugation, a filial series is continued 
in isolation cultures, transferred daily to the same medium as that used for 
the parent series, and parallel records are kept in the same way. A filial 
series, therefore, represents the same original protoplasm as that represented 
by isolation cultures of the parent series. Any difference in the same calendar 
periods between the records of the parent series, and those of the filial series, 
must, therefore, be attributed to the conjugation that has taken place between 
the closely-related cells of the parent series. 
An ideal culture medium for Uroleptus, was found to be an infusion made 
by boiling a small quantity of flour and a small quantity of fine-cut hay in 
spring water. This infusion should be used only after twenty-four hours ex- 
posure to the air. After the first fifty days of experimenting, this culture 
medium, made fresh each day, has been used v/ithout any alterations. 
T have found that the process of 'endomixis' or asexual reorganization 
with restoration of vitality, occurs in Uroleptus mobilis while encysted. Such 
encysted stages persist for long periods, and I find that the organisms cannot 
