236 



GENERAL PHYSIOLOGY 



flowing of the protoplasm into the medium, and a third may follow 

 this, so that the amoeba protrudes its substance in various 

 directions, and thus considerably increases its surface. This 

 extension of pseudopodia, this flowing of substance into the 

 medium, represents the phase of expansion. While a new 

 pseudopodium is being extended, protoplasm usually flows out of 

 another one, from the periphery back to the centre to afford material 

 for the new one, that is, the old pseudopodium is drawn in. This 

 retraction of pseudopodia, this centripetal back-flow of the 

 protoplasm and diminution of the surface associated with it, 

 represents the phase of contraction. If all pseudopodia are drawn 



FIG. 96. A leucocyte (white blood-corpuscle) of the frog, in various stages of movement. (After 



Engelmann.) 



in, the amoeba-cell again assumes a spherical form. The spherical 

 form is, therefore, the expression of most complete contraction in 

 naked protoplasmic masses. When undisturbed, however, simul- 

 taneous contractions and expansions usually take place in the 

 same amoeba at different points on its surface. Hence the 

 pseudopodia are not preformed. Substance flows out, now here, 

 now there, is mixed continually and flows back again, and this 

 changeable play is the amoeboid movement. 



In the various amoeboid protoplasmic masses the form of the 

 pseudopodia varies greatly, according to the special consistency 

 and composition of the living substance. As has already been 

 .seen, 1 there occur among the numerous forms of rhizopod-cells 



1 <?/, p. 76. 



