22 AMEBOID MOVEMENT 



being oval in shape (Figure 3, a) ; but as the pseudopod increases 

 in size, large folds or ridges begin to make their appearance. 

 Usually the first ridges to appear are lateral. They begin as 

 small waves of hyaloplasm which flow out along the sides of 

 the pseudopod for a short distance and then continue to move for- 

 ward. The endoplasm then flows in a number of small parallel 

 streams amid numerous obstructions through the ectoplasmic 

 tube of the pseudopod into the wave of ectoplasm. After the 

 ridge is well begun, there is frequently observed a slow forward- 

 moving stream of endoplasm within it, but the ridge is never 

 closed from the main endoplasmic stream, as is readily proved 

 by the numerous small streams of endoplasm which continually 

 filter through the ectoplasm into the ridge. 



In addition to the lateral ridges, which, as stated, are usually 

 formed first, there appear ridges on the upper side of the pseudo- 

 pod as well, and presumably also on the under side. So far as 

 could be determined these ridges are all formed in much the 

 same way; that is, by the projection of a small wave of protoplasm 

 from some part of the surface of the pseudopod. The ridges do 

 not always grow by extension at the anterior end as described 

 above. Not infrequently a ridge ten to twenty times as long as 

 wide is pushed out along its whole length at once. This is 

 especially likely to happen in a slender pseudopod that suddenly 

 becomes the main pseudopod. The width of a ridge, especially 

 on the upper surface, does not change much after formation. 

 One can frequently find two or three ridges of about the same 

 width, which run the whole length of the ameba with the exception 

 of a short distance at the anterior end, where, as before stated, 

 there are no ridges. 



As the figure indicates, new ridges may be formed from 

 previous ones, either by lateral or endwise extension. In such case 

 the walls of the ridge send out thin waves of hyloplasm followed 

 by streams of endoplasm, as described above in the formation 

 of the first ridge on a pseudopod. When a pseudopod forms a 

 branch, the ridges on the old pseudopod do not likewise branch, 

 but new ridges are formed which have no connection with old 

 ones, but they may later coalesce with old ridges. Such coales- 

 cence is however exceptional. Once a ridge is formed, it retains 



