998 E. H. HARPER 
The inference that the anterior end is heavier is contrary to 
what the shape of the body would indicate, unless the heavier 
particles are located anteriorly. The writer wishes to suggest 
as an explanation of Lyon’s experiment that in strong centrifu- 
gation the same effect is produced at the outset as by mechanical 
agitation, 1.e., the reaction changes to positive. Jensen? showed 
that with weak centrifugation the animals moved centripetally. 
It is conceivable that the pull of gravity on the heavier posterior 
end may produce a tipping effect which is able to orient passively 
but is too weak to stimulate. When, however, the centrifugal 
effect exceeds the pull of gravity and produces too sudden an 
orienting tendency, this may act as a stimulus to the animal to 
resist as in the ordinary rheotropic reaction against the current. 
When strongly centrifuged the animal takes a position so that 
it moves in the water just as the water moves past it in the 
rheotropic response. In other words, if it allowed itself to be 
oriented by the centrifugal force with the posterior end in advance, 
its relation to the water would be the reverse of what it is in the 
rheotropic response. The writer repeated Lyon’s centrifugation 
experiments and the explanation here given, namely, that 
the animal is able to react at the outset of centrifugation, seemed 
to him the ntost satisfactory explanation of the fact that all 
are found to move with the anterior end outward. 
So also in shaking, on account of the difference in buoyancy 
of the two ends, the heavier end will move more rapidly, and this 
may become effective as a stimulus, causing the movement down- 
ward. 
If the explanation here given hold, we have in the normal, 
quiet, geotropic reactions of Paramoecium an example of a purely 
mechanical tropism. The term tropism would be here applied 
to a passive orientation not involving the irritability. If it be 
desirable to use the term ‘tropism’ for such a kind of orientation, 
2Jensen, P. Ueber den Geotropismus niederer Organismen; Arch. f. d. ges- 
Physiol., Bd. 53, pp. 428-480. 
