290 
H. S. JENNINGS 
and this continued during conjugation, multiplication at the 
same time almost ceasing. After conjugation the animals remain 
thin, and usually become fewer in number. After the infusorians 
have remained for some days in this condition, it sufficed to add 
hay anew in order to induce (after a few days) renewed conjugation. 
The precise facts on this series of changes may be illustrated 
from an experiment in which the alterations in size were measured. 
On February 21, 1910, the hay culture of kh had become somewhat 
''old;'' the animals were thin and rather few. This culture 
(which I have denominated on p. 287 as khaa) was derived from a 
single specimen of k, and consisted of individuals in which con- 
jugation had taken place four times in a direct line, as illustrated^ 
in fig. 3. The last epidemic of conjugation had been January 29, 
less than a month before. From this culture I removed a number 
of samples, treating them as follows: 
(a) A quantity of water from the old culture, along with many 
specimens, was placed in a watch glass in a moist chamber. The 
culture fluid being old, these individuals were allowed to hunger. 
(h) A considerable number of individuals were transferred to 
fresh rich ^'standard" hay infusion (1 gram of Timothy hay, 
Phleum pratense, boiled for ten minutes in 100 cc. of tap water). 
This was kept in a watch glass in a moist chamber, along with (a) . 
(Two samples were thus treated) . 
(c) In a third watch glass under the same conditions, individuals 
were placed in a mixture composed of half old culture fluid, half 
of the fresh standard infusion (two samples). 
(d) The original large old culture was divided into two equal 
parts. To one of these a quantity of dry hay was added. 
(e) The last culture consisted of the remaining half of the orig- 
inal old hay culture, to which nothing was added. 
Thus of these five sets, two (a and e) were allowed to hunger, 
while three were placed in a fresh rich infusion. The latter began 
to grow and multiply at once, becoming plump and numerous. 
Two days later (February 23), conjugation began in the freshly 
fed sets h and c, while in the old cultures a and e there was no sign 
of it. In the culture d, to which new dry hay had been added, 
« the animals grew less rapidly, since it requires time for the juices 
