THEORIES OF CHEMOTAXIS AND PHAGOCYTOSIS 221 



they exhibit positive cheinotaxis toward soap solution and 

 other chemicals, the motion being accompanied by current for- 

 mation in the drops. The " pseudopodia," formed by the drops 

 also show currents rushing along their axes and returning by 

 way of the surface. Heat leads to increased activity of motion. 

 The motions were ascribed by Biitschli to the bursting of some 

 of the superficial globules of the foam, which then spread over 

 the surface of the drops, lowering its surface tension at the 

 point of contact. He believed that ameboid motion, likewise, 

 depended upon rupture of surface globules of protoplasm, for 

 the " foam structure " of which he has been the leading advo- 

 cate. 



Bernstein, 1 basing his work on some observations of Paal- 

 zow, observed that a completely inorganic substance, a drop of 

 quicksilver, could be made to imitate ameboid motion under 

 the influence of chemical changes. If near a drop of quick- 

 silver in a nitric acid solution a crystal of potassium dichromate 

 is placed, as soon as the yellow color made by diffusion of the 

 dichromate reaches the drop, the quicksilver begins to show 

 motion and advances toward the crystal. This movement is 

 due to local oxidation of the surface mercury, which lowers the 

 tension on that side of the drop, toward which the mercury 

 then flows. If the crystal is removed, the drop follows, often 

 flowing about it as if to take it in, but soon again withdrawing 

 when the acid dissolves away the oxide formed on the surface, 

 only to return again later. All these movements, which may 

 be very life-like, are readily explained by changes in surface 

 tension that take place under the influence of the bichromate 

 and the acid, and are unquestionably referable to surface phe- 

 nomena. 



Artificial Amebse. By far the most suggestive experi- 

 ments on the simulation of ameboid activity by non-living 

 substances are those of Rhumbler (1898) in his great work, 

 " Physikalische Analyse von Lebenserscheinungen der Zelle." 2 

 On the assumption that the living protoplasm was but a more 

 or less tenacious fluid, following the simple physical laws of 

 fluids, especially in relation to its surface tension, he devised a 

 number of experiments to determine the correctness of these 

 views. An ameba may be regarded as such a mass of viscid 

 fluid, in a medium in which it is nearly or quite insoluble ; it is 

 also constantly undergoing chemical changes within itself, and 

 taking substances from or secreting them into the surrounding 



1 Pfliiger's Arch., 1900 (80), 628. 



2 Arch, f, Entwicklungsmechanik, 1898 (7), 103. 



