i.KOTROPISM OF PARAMKCIUM AND SPIROSTOMIM. 21 



Lyon furnishes evidence (though not conclusive) for his 

 theory as follows: "The animals were strongly centrifuged for 

 several minutes in the luematocrit attachment. Microscopic 

 ( \amination showed that certain dark granules originally dis- 

 tributed were now aggregated in one end, usually the anterior. 

 It is thus seen that differences in specific gravity exist in the 

 protoplasm of this animal" (17, p. 430). 



The writer tried, by means of the centrifuge, to test Lyon's 

 results, but must confess that he could obtain nothing definite. 

 But he thinks now that he ought to have adopted some chemical 

 methods (stains) which might possibly show better results. But 

 since all theories based on the results of external stimulation have 

 been shown above to be fallacious, this theory of Lyon's seems 

 from a physico-chemical point of view to be the most possible 

 and the most reasonable. If this theory is tenable, the matter 

 of the heaviness of either anterior or posterior end is of very little 

 significance in such a unicellular organism as Paramecium, 

 because even the lighter end of the organism may possibly 

 contain protoplasmic materials of a specific gravity greater than 

 that of this region taken as a whole. 



VI. CONCLUSIONS. 



So far as the results of our experiments and analyses are con- 

 cerned, the geotropism of paramecia is due chiefly, if not entirely, 

 to the internal conditions of the organisms rather than to ex- 

 ternal ones. The results of our experiments have shown that 

 the anterior ends of the animals are heavier than their posterior 

 ends. This fact gives no direct clue to our problem, but furnishes 

 ->trong evidence against the mechanical, that is, external, theory. 

 On the other hand, if we suppose with Harper that the posterior 

 ends of the animals are "heavier' than their anterior ends, 

 that the "pull of gravity' "is too weak to stimulate the organ- 

 isms," and that the consequent negative geotropism is due to 

 the orientation of the animals in a "purely mechanical" way, 

 we have no explanation for the lack of uniformity in response to 

 i lie same stimulus under varying conditions. For instance, 

 when given a "shock," the normal paramecia which form a 

 ring at the top of a tube become positively geotropic and swim 



