1 32 COMPARATIVE ANATOMY 



following the advancing hyaline extremity of the pseudopod, 

 until a large lobate process is formed, as large it may be as 

 the whole of the remaining body of the Amoeba. Continued 

 formation of pseudopodia in one direction leads to loco- 

 motion in that direction, but the movement is seldom definite 

 in Amoeba proteus. After any given pseudopod has attained 

 to a certain size it ceases to extend ; the streaming of the 

 granules into its interior is arrested, and eventually the flow 

 is reversed. Another pseudopod has been forming in some 

 other direction, and the granular endoplasm streams off into 

 it, the older pseudopod diminishing in size and eventually 

 becoming obsolete. There are other species however, of 

 which Amoeba Umax is an example, in which progression is 

 more definite. This animalcule does not put forth branching 

 pseudopodia from all parts of its body, but is constantly 

 pushing forward a broad hyaline process from one end of 

 its somewhat elongated body. The granular ectosarc flows 

 forward into this process, and as the latter is continually 

 advancing and being filled by the granular stream from 

 behind, the whole body travels forward in a definite direction. 

 It has been shown by means of a simple experiment that the 

 streaming movement is accompanied by a rolling action of 

 the whole body. If particles of soot are dropped on the 

 Amoeba, some will adhere to its upper surface, and any given 

 particle thus attached may be seen to pass slowly forward to 

 the anterior edge of the advancing animalcule. It passes 

 round this edge to the lower surface and there stops still 

 until the whole body of the Amoeba has passed over it and 

 it has come to lie beneath the posterior end. It then mounts 

 upwards again to the upper surface, is carried forward to the 

 anterior edge, goes underneath it again and so on as long 

 as the animalcule is in movement. Occasionally particles 

 of sand or diatoms are imbedded in the outer layer of 

 the cytoplasm, and these exhibit the same movements as 

 the particles of soot attached to the surface, showing that 

 the ectosarc takes part in the slow rotatory motion. This 

 method of progression can be imitated by partially distending 

 a thin indiarubber bag and rolling it along a table. A spot 

 marked on the upper surface of the bag will be seen to 

 behave like a particle of soot attached to the upper surface 

 of the Amceba. Under certain conditions similar movements 



