THE MECHANICS OF AMOEBOID MOVEMENT 281 



proved 1 , and the long and slender pseudopodia of many animals appear 

 always to have an axial rod of firmer material 2 , which acts as a skeletal 

 framework and appears to be capable of apical growth by the reversible 

 solidification of protoplasm streaming on the surface by the aid of surface- 

 tension energy. Even rapid amoeboid movements may involve changes 

 of cohesion, and the fact that all strong stimuli cause a tendency to 

 the assumption of a spherical shape may point either way. The fact 

 that most cilia are incapable of retraction indicates that they have 

 differentiated into solid organs, and are not liquid protrusions maintained 

 by special conditions of surface-tension. The contractible myoid fibres 

 in the stalk and protoplasm of a Vorticella are also solid structures 3 . 



Rhumbler 4 now adopts the view that amoeboid movements may be 

 aided by changes in the consistency of the ectoplasm, but the possible 

 complexity of the conditions in motile organisms is indicated by the fact 

 that Blochmann 5 found the rapid locomotion of Pclomyxa produced 

 streaming in the surrounding water in the opposite direction to that 

 caused by the movement of a drop of oil towards a soap-solution. 

 According to Blochmann, this is due to the fact that a special streaming 

 movement takes place on the surface of the organism. 



Whether surface-tension or other sources of energy are employed or 

 not, the causes which determine the changes of cohesion still remain to be 

 determined. In addition, surface-tension energy may be brought into 

 play in the interior of the protoplasmic emulsion wherever non-miscible 

 substances are in contact, and in this way much greater total manifesta- 

 tions of energy are possible than when only the external surface-tension 

 comes into play 6 . It is of course always possible that the special surface 

 conditions may directly induce or affect the changes of cohesion in the 

 peripheral layers, and Quincke has shown that surface-tension does affect 

 the formation of precipitation membranes 7 . 



The foam structure which appears to be characteristic of protoplasm 8 

 produces for physical reasons a maximum consistency with a minimum 

 of material 9 , but does not cause the endoplasm to lose its fluid character. 

 The solidity of the mass increases as the emulsion becomes finer, but the 

 rise of cohesion in Myxomycetes does not appear to be produced in this 



1 Rhumbler, Zeitschr. f. allgem. Physiol., 1898, p. 195. * Id., p. 114. 



3 Cf. Verworn, Allgem. Physiologic, 1901, 3. Aufl., p. 252. 



4 Rhumbler, I.e., 1903, Bd. II, p. 315. 



5 Blochmann, Biol. Centralbl., 1894, Bd. xiv, p. 82. 



6 Cf. Ewart, Protoplasmic Streaming in Plants, 1903, p. 112 seq. 



7 Quincke, Annalen der Physik, 1902, Bd. vn, pp. 631, 701. 



8 Cf. Biitschli, Archiv f. Entwickelungsmechanik, 1901, Bd. xr, p. 499; Rhumbler, I.e., 1903, 

 Bd. II, p. 326. 



8 Lehmann, Molecularphysik, 1884, Bd. I, p. 257; Quincke, Ann. d. Physik, 1894, Bd. Lin, 

 p. 616 ; 1902, Bd. xvil, p. 639. 



