144 TROPISMS 



slide will be nearly deserted. One of the first investigators to describe 

 this phenomenon was Pfeffer. He observed its occurrence in Glaucoma 

 scintillans, and less markedly in Colpidium colpoda, Stylonychia mytilus, 

 and Paramcecium. Pfeffer was inclined to believe that these aggregations 

 were due, partly at least, to a contact stimulus, resulting from a striking 

 of the organisms against small solid bodies, and especially against each 

 other. 250 



This conclusion of Pfeffer may after all be correct, 

 since it has been shown that sea water containing jelly 

 from the egg of a sea nrchin causes spermatozoa to stick 

 together for some time when they impinge upon each 

 other. This agglutination no longer occurs when the 

 spermatozoa are immobilized. Jennings came to the con- 

 clusion that these aggregations of infusorians are due to 

 the fact that they can go into a weak concentration of 

 acid, while they cannot escape from such a weak concen- 

 tration; and since Paramcecia themselves produce C0 2 

 he assumed that the C0 2 produced by themselves acts as 

 a center of attraction for other Paramcecia. In order to 

 prove this he used the following method : 



The organisms were studied in a thin layer of water, by mounting 

 them on a slide covered with a large cover glass supported near its 

 ends by slender glass rods. Their reactions were tested by introducing 

 with a capillary pipette a drop of the substance in question beneath 

 the cover glass, or in some cases by allowing it to diffuse inward from 

 the side of the cover glass. 250 



Thus Jennings introduced a drop of 1/50 per cent. 

 (m/180) HC1 on a slide containing Chilomonas. Very 

 soon a somewhat denser ring of these individuals was 

 formed around the drop (Fig. 41). A 1/50 per cent. HC1 

 solution paralyzes (and soon kills) Chilomonas or Para- 

 mcecia and hence the surface of the drop must act like 

 a trap into which the organisms will steadily swim, with- 

 out being able to sw T im back. This will naturally increase 



