70 AMEBOID MOVEMENT 



attached to the substratum.) Sphaeronucleosiis stands in contrast 

 with discoides for it is attached to the substratum over a much 

 greater area and in consequence only a slight amount of surface 

 is drawn from the under side. This ameba may therefore be 

 regarded in this connection as of only one surface, the upper. 

 That part of outline i in Figure 14 cut by outline 2 indicates, as 

 in discoides, the region of forming ectoplasm, and the space 

 between outlines i and 2 may be used as a basis of computation. 

 New ectoplasm is formed in this zone and far enough back to 

 include the tips of the longitudinal ridges, of which we have 

 already spoken (Figure 13). The zone of forming ectoplasm 

 would therefore be about twice as wide as the average width of 

 the three zones between the successive outlines in the figure, and 

 of approximately the same shape. On this basis, the surface oc- 

 cupied by forming ectoplasm is 1/5.8 of the total surface, and the 

 ratio of formed to forming ectoplasm is 4.8 to i. 



(For the sake of completeness, a few factors whose values cannot 

 easily be computed may be mentioned, i. The anterior edge is 

 not attached to the substratum at its farthest point, but at some 

 little distance back of the edge, thus increasing the relative amount 

 or forming ectoplasm ; but this is offset by the surface of a part of 

 the under side at the posterior end where the surface layer is 

 active. 2. The ectoplasm composing the ridges, which must be 

 added to the formed ectoplasm, would increase the ratio, though 

 only slightly). 



Approximately twice as much ectoplasm is therefore in the 

 process of formation in sphaeronucleosus as in discoides when 

 compared with the formed ectoplasm in the respective amebas, 

 over which the surface film is active. This ratio corresponds 

 very well with the rate of movement of the outer surface in these 

 amebas, which as we have seen is about twice as fast in sphaero- 

 nucleosus as in discoides. 



Where does the surface layer come from and what becomes 

 of it after it arrives at the anterior end? It moves continually 

 forward as long as the ameba moves forward. There would 

 seem to be a tendency therefore for it to accumulate at the end 

 of a free pseudopod in such a form as discoides, and even under 

 ordinary conditions of locomotion where there is occasional at- 



