On the Mechanism of Ameboid Movement *

SYNOPSIS. Current theories for explaining movement in Amoeba are the contraction‐hydraulic and the fountainstreaming theories. The first assumes that plasmasol is pushed by hydraulic pressure developed by contraction of posterior gel. The second assumes that plasmasol is pulled by a condensation process at the anterior end of the gel tube. By means of time exposures (5–10 seconds) under dark‐field illumination it has been possible to obtain direct photographic traces of the movement of particles of the sol and of the gel. These demonstrate that: 1) Some advancing pseudopods display no background movement of particles at the tip. 2) Some granules move straight to the very tip in the central portion of an advancing pseudopod, and do not curve to either side until later. 3) In one withdrawing pseudopod, all of the granules of the gel moved simultaneously inward and perpendicularly to the flowing plasmasol along the entire length of the pseudopod. 4) In the anterior portion of the pseudopod the large granules of the gel may move forward. Furthermore, 5) There is a general anteriorly directed fluid flow detectable by observation of very fine granules through the interstices of the gel tube and also through the hyaline ectoplasm. 6) There is a forward flow throughout the entire cross section of the anterior portion of an advancing pseudopod, namely gel, interstices of the gel, sol, ectoplasm, and plasmalemma. It is concluded that the fountain‐streaming theory is incompatible with the above facts, and that these can be explained by the contraction‐hydraulic theory.