170 The Structure of Protoplasyn 



fountain streaming as in locomotion and as Mast (1926, p. 407) 

 points out "could in no way be involved in the forward extension 

 of the pseudopod, resulting in locomotion." The cessation of stream- 

 ing of the plasmasol by pressure is thus probably due to the decrease 

 in the viscosity of the plasmagel. 



LOCOMOTION OF AMEBA 



The locomotion of Ameha proteus can be considered as the type 

 representative of ameboid locomotion. The classical papers of Mast 

 on the structure and locomotion of Ameha proteus contain the most 

 important account of this. They contain a detailed study of the 

 structure which is essential for any understanding of locomotion 

 and of the various factors involved. Every visible detail, including 

 those seen with the highest powers of the microscope, is important 

 as are also all the changes which occur from moment to moment 

 during a period long enough to cover a complete cycle. Even with 

 the most complete record of the visible structure and all the visible 

 changes it undergoes in locomotion or any other process, one comes 

 immediately to the invisible structures and processes which are still 

 more fundamental, but unfortunately they are still obscure. 



Mast's account of the structure and locomotion of the Ameha 

 proteus is too well known for me to consider it in any great detail. 

 The moving ameba has a plasmogel layer surrounding a fluid plasmo- 

 sol. The gel layer is thick over the sides of the tubelike body, some- 

 what less thick at the posterior end, and thin over the anterior 

 pseudopod. The outer plasmolemma and the necessity of a sub- 

 stratum need not be considered. Contraction of the plasmogel at 

 the posterior end drives the plasmasol forward and extends the 

 pseudopod at the weak anterior end. As the posterior end of the gel 

 layer contracts, it shortens and loses material because part of it 

 keeps going into solution. This sol mixes with and is carried forward 

 in the plasmosol. The posterior end is thus gradually shortened. 

 At the anterior end some of the plasmosol goes to the lateral wall of 

 the pseudopod and there gels as it comes in contact with the 

 anterior end of the thick plasmogel tube wall. The tube wall is 

 built up anteriorly as rapidly as it solates posteriorly. The follow- 

 ing significant observation by Mast (1923) is readily explained by 

 the above considerations, but not by any of the other explanations 

 of ameboid locomotion, (p. 259) "If a given granule in the plasmosol 

 is continuously observed it can be seen to move forward in the 

 neighborhood of the central axis of the ameba until it reaches the 



