Amoeboid Movement 



633 



theories attribute this to the elastic strength or contractileness of the plas- 

 magel, which is greater posteriorly and squeezes the plasmasol forward. 



Lateral examination of granules in an amoeba and of particles adhering to 

 the plasmalemma has shown that, as the cell advances, granules in the gel 

 remain fixed until the posterior end reaches them, when they enter the 

 sol (Fig. 240). Similar stability of the gel is shown in advancing lympho- 

 cytes, in which small lateral protuberances remain fixed until the posterior 

 end reaches them.^ Particles on the ventral attached portion of the plasma- 

 lemma are also fixed, but particles elsewhere on the plasmalemma, both 

 dorsal and ventral, move forward (Fig. 240). Thus the plasmalemma cither is 

 stretched and varies in thickness, or is rapidly constituted at the posterior 

 end and in freely extending pseudopods.^^ Mid-dorsally a granule may 

 move faster than the rate of advance of the amoeba, indicating active slip- 

 ping of the plasmalemma over the hyaline layer. ^^ In A. verrucosa, rarely 

 in A. proteus or A. Umax, there is a "rolling movement," the plasmalemma 

 rolling like a rubber bag of liquid down an incline.-^^' ^^ When attachment 

 is discontinuous there may be a "walking" movement; the advancing tip 



Fig. 240. Series of sketches of Amoeba protens as seen from the side, representing the 

 movement of oifferent parts of the body during locomotion. A, B, C, Three successive 

 positions on the same substratum xy; d, e, f, g, i, particles in the plasmagel and the 

 plasmasol; a, h, c, particles attached to the plasmalemma. From Mast." 



attaches and by contraction of gel just behind the tip the cell is pulled for- 

 ward. This is well shown by Difflugia, which has a shell consisting of sand 

 grains cemented together. One pseudopod extends freely, waves about, and 

 finally attaches at its tip, then as the gel of the first pseudopod contracts, 

 a second pseudopod is extended, receiving plasmasol from the attached pseu- 

 dopods, which thus shortens and pulls the shelled portion forward.'^- ^^ 



Freely crawHng amoebae move at rates of 0.5 to 4.5 micra per second, 

 most of them at the rate of about 1 micron per second.^- Monopodal indi- 

 viduals travel faster than multipodal ones; monopodal specimens of A. proteus 

 in 0.001 N NaCl averaged 4.6, irregular monopodal 4.3, and multipodal 2.1 

 micra per second.^^ Lymphocytes in tissue culture moved an average of 0.55 

 |ii/sec., whereas the non-polarized macrophages averaged 0.004 /x or less per 

 second.^ 



