THE MECHANICS OF AMOEBOID MOVEMENT 279 



due to changes of surface-tension when the protoplast is freely suspended 

 in a liquid, but in other cases the cohesion of the peripheral layers may 

 more or less counteract the direct action of surface-tension. In Myxo- 

 mycetes, for instance, the ectoplasm appears to be more solid in consistency 

 than the central endoplasm, and it forms a layer of variable thickness 

 and properties. Changes of consistency may often play an important part 

 in amoeboid movement, as well as in the formation of cilia, and many 

 protoplasts appear able to" raise their consistency when necessary \ or to 

 render themselves solid like gelatine by forming a slender framework of 

 solid substance in which the more fluid materials are embedded. The 

 cell-wall is in fact a peripheral skeletal structure formed either by external 

 secretion or by protoplasmic metamorphosis. All stages of transition may 

 be shown between a viscous liquid and a colloidal solid, just as during the 

 solidification of melted gelatine. 



Since the protoplast is able to dissolve away its cellulose investment 

 in case of need, it is not surprising that the increased consistency of the 

 protoplasm should be capable of decrease, as appears to be the case 

 in Myxomycetes. When this occurs, surface-tension again becomes of 

 predominant importance, and may be able to produce the retraction 

 of prominences formed in the more solid condition. In case the expansion 

 and contraction are produced by an antagonism of this kind, a pseudo- 

 podium may be produced without any change of surface-tension, whereas 

 otherwise a very strong depression of surface-tension would be required. 

 No definite conclusions can be made as yet, for the cohesion and viscosity 

 of the protoplasm cannot at present be accurately determined, and in 

 addition it is not known to what extent the protoplast may raise or lower 

 its general surface-tension. 



Even when a particular mechanical action has been proved to be due 

 to surface-tension energy or some other agency, it still remains to be 

 determined how the supplies of energy are controlled and utilized, and 

 how the conditions for their action are produced. Apart from its physical 

 action surface-tension energy and like forces may act as stimuli and induce 

 special responses. It is easy to see how the firmer ectoplasm of the 

 plasmodia of Chondrioderma, Aethalium, and other Myxomycetes is 

 produced from fluid endoplasm, and may be reconverted into the latter 2 . 

 The ectoplasm may be ooi mm. thick, and is, therefore, more than a mere 

 surface-tension film, and is much thicker than the ectoplasmic membrane. 

 Its production is the direct result of its peripheral position, and similarly 

 the cell-wall and the ectoplasmic membrane are only formed on the surface 



1 Cf. Pfeffer, Zur Kenntniss der Plasmahaut u. d. Vacuolen, 1890, p. 255 ; Rhumbler, Zeitschrift 

 f. allgem. Physiologic, 1902, Bd. I, p. 281. 



* Pfeffer, Zur Kenntniss der Plasmahaut u. d. Vacuolen, 1890, p. 256. 



