THE GEOTROPISM OF PARAMOECIUM 995 
Many factors influence the movements of Paramoecia so that 
some cultures aggregate quickly at the top in a ring and others 
remain distributed, and in some cases move downward. The 
various unknown factors have been disregarded and comparisons 
made only to determine whether the treated specimens exhibited 
a stronger negative geotropic tendency than the ones not treated. 
The placing of iron filings in the control tube was to eliminate 
any possible chemotactic factor. Finely divided bismuth and 
nickel were also tried, but iron was found entirely satisfactory 
for the purpose of the experiment. 
One condition of the experiments is that the iron filings after 
a time become distributed through the endoplasm more evenly, 
so that any difference in behavior is to be looked for before this 
change occurs. A second condition is that an excessive amount of 
iron may overload the animals apparently and cause them to 
aggregate at the bottom. The Paramoecia rid themselves of 
the iron after the course of a few hours without apparently harm- 
ful effects from a small amount. 
One more precaution needs to be stated. The whole quantity 
of iron in the mortar must be removed with the Paramoecia, or 
a possible loss might occur of some individuals whose tendency 
was to remain on the bottom, or else the iron must be repeatedly 
rinsed to remove all of them. 
The experiments have been repeated so often that certain 
results appear as typical, and these will be reported just as 
recorded. 
Experiment 1 Control tube referred to as No. 1; treated indi- 
viduals as No. 2. Ingestion of iron for five minutes. 
Five minutes after the beginning of the experiment the distri- 
bution in the two tubes did not noticeably differ. 
In ten’ minutes there was about an equal collection gathered 
at the bottom of each tube. To observe the size of these aggre- 
gations better the tubes were slightly shaken. 
Twenty minutes later there was a large collection at the top 
of No. 2, very few at the top in No.1. Looking down from above 
gave the appearance shown in fig. la; fig. 1b gives a side view o 
the same. 
JOURNAL OF MORPHOLOGY, VOL. 22, NO. 4 
DECEMBER, 1911 
