996 E. H. HARPER 
In this experiment, on account of the long treatment, a con- 
siderable number had apparently taken on an overload of iron, 
which would account for the aggregation at the bottom of No. 2. 
In one hour and twenty minutes nearly all had collected at 
the bottom in both tubes, but a collection still remained at the 
top in No. 2, none in No. 1 (fig. 2) 
Experiment 2. Animals from the same culture, same day. 
Ingestion of iron for thirty to forty seconds. 
Fifteen minutes later, there was an aggregation in the bottom 
of No. 1, and afew scattered all the way up the tube In No. 2 
there was a slight aggregation at the bottom and a considerable 
collection above the middle of the tube. 
In twenty minutes the collection appeared as shown in fig. 3. 
The smaller number at the bottom of No. 2 was referable to the 
fact that less iron had been ingested than n the former experi- 
ment Notwithstanding the normal tendency to go downward, 
there was a decided tendency among the treated animals to go 
upward. 
Experiment 3. This is a sample of those experiments with 
cultures in which the normal tendency was to go upwards and 
form a more or less dense ring at the top of the tube. Ingestion 
of iron for one minute. In ten minutes a much denser ring was 
formed at the top of No. 2, and this inequality remained (fig. 4). 
The Paramoecia in No. 2 moved more slowly than in No. 1. 
When the test tubes were corked and inverted, no air being al- 
lowed to enter, there was the same collection at the top, but not 
ring formation. 
The interpretations of Experiment 3 might differ. The denser 
ring at the top of No. 2 might be explained as an entrapping of 
the animals in this region, resulting from their slower movements, 
on the principle by which Jennings explains aggregations due to 
chemical influences. At any rate the fact remains that the 
treated animals swarmed to the top more quickly and formed a 
denser ring there than in the control. These experiments are 
less crucial than those with animals having a normal tendency 
to go downward. 
