§ 5] THEEMOTAXIS 259 



in the rays of the infallijig, concentrated light of midsummer, 

 reflected from the mirror. Upon the black paper was placed 

 the cover-glass with the amceba in a drop of water. The light 

 from the mirror was cut off until the amoeba, in its migrations, 

 lay half-way over the edge of the orifice. Then concentrated 

 light was let through the slit. A small part of the body was 

 still moved across tlie line of demarcation; then for a moment 

 movement ceased and a few seconds after the protoplasm of the 

 amosba began to flow backwards. In from 10 to 30 seconds 

 the amoeba was wholly in the dark again. Similarly, when the 

 cover-glass was moved so that the amoeba was brought half-way 

 over the open orifice it retreated into the dark. Direct measure- 

 ment showed that the temperature at the illuminated part was 

 40° to 60° C, whilst over the black paper it was 15° to 20° less. 

 That the movement was not due to the light was shown first 

 by cutting out, by means of ice, the beat rays only. No reac- 

 tion occurred. Secondly, by cutting out the light but not the 

 heat, by passing the light through a solution of iodine in CS^ 

 so that only the ultra-red rays (which act like darkness to all 

 organisms) went through; the typical reaction occurred when 

 the temperature over the slit was 35°. From all of these ex- 

 periments the conclusion seems justified — Amoeba is positively 

 thermotactic towards that temperature. 



Similar results were obtained by Vervs^oen with the shelled 

 Rhizopod Echinopyxis aculeata, and later (see Jensen, '93, 

 p. 440) with Paramecium. More complete studies on the 

 latter were, however, made by Mendelssohn, who worked in 

 Veeworn's laboratory. Mendelssohn devised an excellent 

 method of study. A brass plate 20 cm. x 6 cm. and 4 mm. 

 thick is properly supported in a horizontal position, and to its 

 under face are affixed, transversely, tubes through which hot 

 Or cold water jnay be run from a reservoir placed at a high 

 level. In the middle of the plate a space 10 cm. x 2 cm. and 

 2 mm. deep is cut out and into it is fitted a glass or ebonite 

 trough. Special thermometers whose bulbs are coiled in the 

 plane of the trough, and hence perpendicularly to the stem, 

 serve to measure the temperature of the water in the trough at 

 any point. By means of water running through the transverse 

 tubes either end of the trough may be heated or cooled as 



