THE MECHANISM OF LIFE 



561 



in proportion to the mass, and one of expansion, in which the 

 surface is increased. In amoeboid movement expansion is expressed 

 in the extension of pseudopodia, and contraction in their retraction 

 and the endeavour to assume a spherical form (Fig. 268). The 

 interchanges between the two constitute the whole phenomenon of 

 amoeboid movement. As is well known, a naked protoplasmic 

 drop, for example an Amceba-ce\l, behaves physically like a liquid. 

 Its movements must, therefore, obey the general laws of liquids, as 

 Berthold ('86) especially has consistently applied them to numerous 

 special cases. Physically considered, every movement of a drop 

 of liquid is the expression of changes of surface-tension, i.e., of 

 the energy of cohesion with which the individual particles in a 

 freely-suspended drop attract one another. If the surface-tension 

 is equal at all points, the drop assumes a spherical form. If for 

 any reason it is diminished in one place, there occurs there as the 

 result of pressure from the other sides a protuberance which in- 

 creases until equilibrium 

 is again established. If 

 the surface-tension at the 

 same place becomes 

 greater, the protuberance 

 diminishes correspond- 

 ingly. Hence, the spheri- 

 cal form of an amoeboid 

 cell is the expression of 

 a surface-tension equal at 

 all points ; the extension 

 of pseudopodia at local-, 

 ised places is the index of 

 a diminution of surface- 

 tension at those places. 



In other words, the problem of amoeboid movement thus made clear 

 is contained in the question : what causes, on the one hand, a 

 diminution of surface-tension (extension of pseudopodia), and, on 

 the other, an increase of surface-tension (retraction of pseudopodia 

 and tendency toward a spherical form) ? 



Regarding the manner of diminution of surface-tension, Ktihne's 

 experiments ('64) upon Amoeba and Myxomycetes, already spoken of, 

 are decisive. When Kiihne placed a drop containing Amcebce in 

 a medium that contained no oxygen, but was indifferent in other re- 

 spects, as, for example, hydrogen, the amoeboid movement gradually 

 ceased, and the Amcebce maintained the forms that they had 

 already assumed. If oxygen were then allowed to enter, the move- 

 ment began again, new pseudopodia were extended and the Amcebce 

 resumed their creeping. Kuhne's experiments upon the plasmodia 

 of Myxomycetes are equally clear. He put a lump of a dry 

 plasmodium of Didymium in a vessel filled with water containing 



o o 



FIG. 268. Amoeba in outline; the nucleus lies in the 

 interior. A, Extending pseudopodia in various 

 directions ; S, creeping in one direction 

 tracted into a ball. 



C, con- 



