CHAP. ii. J THE CONTRACTILE TISSUES. 137 



Amoeboid Movements. 



87. The white blood corpuscles, as we have said ( 28), are 

 able of themselves to change their form and by repeated changes 

 of form to move from place to place. Such movements of the 

 substance of the corpuscles are called amoeboid, since they closely 

 resemble and appear to be identical in nature with the movements 

 executed by the amoeba and similar organisms. The movement 

 of the endoplasm of the vegetable cell seems also to be of the 

 same kind. 



The amoeba changes its form (and shifts its place) by throw- 

 ing out projections of its substance, called pseudopodia, which 

 may be blunt and short, broad bulgings as it were, or may be so 

 long and thin as to be mere filaments, or may be of an intermedi- 

 ate character. As we watch the outline of the hyaline ectosarc 

 we may see a pseudopodium beginning by a slight bulging of the 

 outline; the bulging increases by the neighbouring portions of 

 the ectosarc moving into it, the movement under the microscope 

 reminding one of the flowing of melted glass. As the pseudo- 

 podium grows larger and engages the whole thickness of the 

 ectosarc at the spot, the granules of the endosarc may be seen 

 streaming into it forming a core of endosarc in the middle of the 

 bulging of ectosarc. The pseudopodium may continue' to grow 

 larger and larger at the expense of the rest of the body, and 

 eventually the whole of the amoeba including the nucleus may as 

 it were have passed into the pseudopodium ; the body of the 

 amoeba will now occupy the place of the pseudopodium instead of 

 its old place ; in other words it will in changing its form have also 

 changed its place. 



During all these movements, and during all similar amoeboid 

 movements, the bulk of the organism will, so far as can be 

 ascertained, have remained unchanged ; the throwing out a pseu- 

 dopodium in one direction is accompanied by a corresponding 

 retraction of the body in other directions ; if as sometimes happens 

 the organism throws out pseudopodia in various directions at the 

 same time, the main body from which the pseudopodia project is 

 reduced in thickness ; from being a spherical lump for instance it 

 becomes a branched film. The movement is brought about not 

 by increase or decrease of substance but by mere translocation of 

 particles ; a particle which at one moment was in one position 

 moves into a new position, several particles thus moving towards 

 the same point cause a bulging at that point, and several particles 

 moving away from the same point cause a retraction at that 

 point ; but no two particles get nearer to each other so as to 

 occupy together less space and thus lead to condensation of sub- 

 stance, or get farther from each other so as to occupy more space 

 and thus lead to increase of bulk. 



