452 



GENERAL PHYSIOLOGY 



differences of temperature always exist at two different points in 

 the medium in the same direction from the source. 



The iirst observation of thermotactic properties was made by 

 Stahl ('84) in plasmodia of Aethalium septicum. He placed two 

 beakers side by side, one of which was filled with water at a 

 temperature of 7°, the other with water at 30°; he then laid over 

 their edges a strip of filter-paper, upon which the plasmodium had 

 spread itself out, in such a manner that one end of the j^lasmodium 

 dipped into the colder, the other into the warmer water. The pro- 

 toplasm of the plasmodial network at once began to stream out of 

 the former toward the latter, although before the experiment the 

 opposite direction had been followed. The whole protoplasmic 



-^msa 



Fig. 226. — Negative therniotaxi.s of Amaha. I, A drop of water coutaining many amoebae lies upon 

 a large cover-glass. The cover-glass lies upon a black ground, which has in the middle a sharp, 

 square opening. By shoving the cover-glass an amoeba can be so placed that it creeps over 

 the edge of the hole, //, A. If concentrated sunlight is then let through the opening from the 

 mirror of the microscope, the amoeba creeps back immediately into the cool darkness, //, B. 

 The arrows indicate the direction of the movement. 



mass finally passed over to the warm water. This is a case 

 of positive thermotaxis. 



Negative thermotaxis can be observed in Aiiueba} when a 

 temperature of at least 35°C. is allowed to act upon one part of 

 the body while the rest of the protoplasm is at a lower temperature. 

 This can hardly be accomplished by means of conducted heat. 

 Radiating heat and the following arrangement should be em- 

 ployed. A large drop of water, containing many individuals of 

 Avuaba Umax, is placed upon a large thin cover-glass and the 

 latter is laid upon a glass plate cemented to black paper and placed 

 upon the stage of the microscope. In the middle of the paper is a 

 small liolc with very sharp edges. The concave mirror of the 



1 Cf. VeiM'orn ('89, 1). 



