CHAPTER XI 

 THE SURFACE LAYER AS A LOCOMOTOR ORGAN 



The discussion of the surface film of ameba and its movements 

 during locomotion naturally led to a discussion of the various 

 theories that have been offered to explain ameboid movement 

 and protoplasmic streaming. Now the fact that the ameba pos- 

 sesses a traveling surface film which can carry particles 'recalls 

 similar behavior in Oscillatorias and in the diatoms. No new 

 observations have been made very recently, but by comparison 

 of the behavior of particles carried by an Oscillatoria filament 

 and by ameba, it is found that the nature of the movement, the 

 rate of movement, the degree of adhesion of the particles, the sizes 

 of the particles carried and so on, are similar in both organisms. 

 This indicates that there is a surface layer on Oscillatoria threads 

 that is similar to that which has been described in amebas, and 

 whose movement is probably also effected by changes in surface 

 extension ; but just how this change is effected is not clear owing 

 to the spiral path the particles take as they travel along the Oscilla- 

 toria filament. The spiral has an angle of about sixty degrees, 

 which must be related in some way to the finer structure of the 

 cells of which the filament is composed. The suggestion that move- 

 ment is caused by the rapid and forcible exudation of mucus is ex- 

 ceedingly improbable if not physically impossible. It is difficult to 

 see how the spiral direction of the flow of mucus could be brought 

 about, to say nothing of the frequent change in direction of the 

 flow. In a surface tension film, however, the direction of move- 

 ment is readily determined by the location of the points where the 

 tension is changed. Mucus secreting glands would need special 

 structural devices such as secretory tubes bent at an angle to 

 control the direction of flow, while no such structural devices 

 are necessary if the propelling force is surface energy. In short, 

 it is difficult to see how any movement at all could be produced 

 by the act of secretion of mucus, while from what we have seen 

 in the ameba, surface tension changes could easily produce move- 



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