68 AMEBOID MOVEMENT 



being made, is proportionately much wider in sphaeronucleosus 

 than in discoides as compared with the amount of surface back 

 of it. Figures 23 and 24 show that the rate of movement of the 

 surface film is directly proportional to the amount of new ecto- 

 plasm forming. In the second place, the greater part of the under 

 surface in the forward half of sphaeronucleosus is attached to 

 the substrate, so that the surface layer which flows toward the 

 anterior end is derived almost wholly from the upper surface; 

 while in discoides the whole surface in free pseudopods, and 

 nearly the whole surface in attached amebas (cf. Bellinger's ob- 

 servations described on p. 56) possesses mobile surface proto- 

 plasm. Observation of moving particles on these amebas proves 

 this. Then again, the anterior edge of a sphaeronucleosus is not 

 attached at the points farthest advanced, but the point of attach- 

 ment is some distance back, as indicated in figure 20. The effect 

 of this is to increase the amount of forming ectoplasm in propor- 

 tion to the surface of the ameba from which surface protoplasm 

 may be drawn. Still one other factor must be considered. As 

 is well known sphaeronucleosus, -uerrucosa and their congeners 

 possess longitudinal ridges on the upper surface which consist 

 of ectoplasm, covered of course by the surface film. These ridges 

 are formed near the anterior edge, not by wrinkling, but by the 

 construction of new ectoplasm. Once formed, they remain 

 until the ameba, so to speak, flows out from under them. That 

 is, the ridges undergo comparatively slight changes until changed 

 back into endoplasm at the posterior end of the ameba. As the 

 ameba flows ahead the ridges are of course continually being 

 added to or lengthened, by the conversion of some endoplasm into 

 ectoplasm. The ridges may thus retain their identity for a long 

 time although the substance composing them is changed every 

 time the ameba moves the length of its body. It is clear, there- 

 fore, that there is more ectoplasm formed at the anterior end of a 

 spliaeronufleosus than would be the case were the upper surface 

 of the ameba plane; and the conclusion therefore is obvious 

 that the formation of ridges, occurring as it does, chiefly at the 

 anterior end, serves further to accelerate the forward movement 

 of the surface film. 



If the form of sphaeronucleosus were more regular than it is, 



