AMEBOID MOVEMENT 27 



stream). But because of friction against the sides of the ecto- 

 plasmic tube, there is a layer of endoplasm of appreciable thick- 

 ness that is practically motionless. This layer of endoplasm 

 therefore makes the diameter of the endoplasmic stream appear 

 smaller than it actually is, and the ectoplasmic tube larger than 

 it is. The actual thickness of the tube of ectoplasm, as distin- 

 guished from the flowing endoplasm, is difficult to measure, but it 

 seems to be about one-tenth the diameter of the pseudopod. 

 (Kite ('13) found ameboid ectoplasm to be from eight to twelve 

 microns thick, but he does not state from what part of the ameba 

 nor from what species the ectoplasm was taken.) This would 

 indicate that if the transformation of endoplasm into ectoplasm 

 is as complete as the conditions permit, the thickness of the fric- 

 tion layer would be about one-twenty-third of the diameter of 

 the pseudopod. These observations therefore point to the con- 

 clusion that the tendency in laureata is for all the endoplasm to 

 be transformed into ectoplasm at the anterior end, and for the 

 reverse process to occur at the posterior end. 



Several of the pelomyxas also move in much the same manner 

 as Amoeba laureata, that is, in clavate form and more or less 

 cylindrical in shape. This is especially the case with Pelomyxa 

 palnstris and P. belevskii. But in these species the endoplasm is 

 not completely converted into ectoplasm at the anterior end, as 

 is shown by the fact that there is a slight backward current of 

 endoplasm at the sides near the anterior end (Schultze, '75). Ob- 

 servation indicates also that the ectoplasmic tube is thinner than 

 would be the case were there complete transformation of endo- 

 plasm into ectoplasm at the anterior end. The origin of pseudo- 

 pods in these pelomyxas is not steady and under control as in 

 laureata, but sudden and eruptive, indicating a less coherent ecto- 

 plasm. 



The nearest approach to the conditions of streaming as found in 

 Amoeba laureata is found in A. discoides (Figure n, B) a species 

 often confounded with proteiis'. This species is frequently found 

 in clavate form, and the conversion of endoplasm into ectoplasm 

 is complete at the anterior end. In other respects of streaming 

 and pseudopod formation, the two species are also similar. 



In another very common species of ameba, Amoeba dubia 



