CHAPTER 17 



Amoeboid Movement 



ROTOPLASMic STREAMING occurs in many living cells, possibly in all. 



Diffusion is too slow a process for the transport of solutes from one part 

 of a cell to another, and active cytoplasmic movement provides a 

 rapid transport mechanism. Streaming may proceed in a fixed path and may 

 be fast enough for direct microscopic observation, as the cyclosis around the 

 vacuole of some plant cells, or the transport of food vacuoles and granules 

 about the body of a ciliate protozoan. In other cells streaming may be slow 

 and may be more of a churning than a fixed current; such cytoplasmic activity 

 is best seen by accelerated motion pictures, as of tissue cultures (fibroblasts, 

 etc.-^), or of tips of growing nerve fibers.''"- ''•^ The cellular mechanism of 

 protoplasmic streaming is unknown. Its dependence on colloidal properties, 

 particularly on viscosity changes, is shown by the effects of hydrostatic 

 pressure; streaming in Elodea is slowed in proportion to a decrease in 

 viscosity.-^ 



TYPES OF AMOEBOID CELLS AND OF PSEUDOFODS 



Amoeboid movement has a colloidal basis, as does protoplasmic stream- 

 ing; amoeboid movement is also accompanied by changes in cell shape and 

 often by progressive motion. Amoeboid movement may be directed locomo- 

 tion, as in the rhizopod Protozoa (Sarcodina), in the plasmodium of myxo- 

 mycetes, in amoeboid leucocytes (particularly vertebrate lymphocytes), in 

 amoeboid or wandering cells of many kinds of animals (such as the archeo- 

 cytes of sponges), and in growing nerve fibers; or amoeboid movement may 

 consist in the extension, flexion, and retraction of processes (pseudopods) 

 concerned primarily in feeding, as in most Foraminifera, Heliozoa, Radio- 

 laria, vertebrate macrophages, and various phagocytes such as reticuloendo- 

 thelial cells (e.g., Kupffer cells of the liver). Locomotory amoeboid move- 

 ment requires attachment to some substrate; non-polarized amoeboid move- 

 ment occurs in free pseudopods. 



Among the free-living amoeboid animals the manner of locomotion differs 

 slightly according to cell form and type of pseudopod. Pseudopods may be 

 lobopods, broad to cylindrical and round at the tip (Fig. 239, A-E); they 

 may be filopods, slender with pointed tips; they may be reticulopods, thread- 

 like, branching, and anastomosing as in Foraminifera, or they may be axo- 

 pods, rays with a central axial rod as in Heliozoa and Radiolaria (Fig. 

 239, G).'" Locomotion by lobopods has been much studied; filopods, reticu- 

 lopods and axopods are primarily freely extended feeding pseudopods; filo- 

 pods may show streaming without accompanying change in length, and axo- 

 pods may be very contractile. 



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