234 GENERAL PHYSIOLOGY 



in expansion, so that the same mass presents a larger surface. 

 Transition from one phase to the other alone renders possible 

 phenomena of motion. 



It is evident that only bodies of more or less liquid consistency 

 can show such movement ; only a liquid can diminish or increase 

 its surface by rearrangement of its particles, becoming spherical 

 or spread out, according as its surface-tension is equal in all 

 directions, or becomes greater in some places and less in others. 

 A solid, stiff body, even if it is elastic, cannot manifest con- 

 traction-phenomena of this kind, because its particles cannot 

 change their mutual positions. Hence, it is of fundamental im- 

 portance for the occurrence of contraction-phenomena that living 

 substance possess a liquid consistency. As a matter *of fact, all 

 living substance, as has already been found, is more or less liquid, 

 a condition that is imposed upon it by the high percentage of 

 water in its contents, and, therefore, the common view is well 

 founded that all living substance possesses contractility, although 

 many cells are known, such as certain Algce and Bacteria, which 

 in spite of their leading an active life can perform no contraction- 

 phenomena, because they are surrounded by a stiff membrane. 

 Contractility, i.e., the property of executing contraction-move- 

 ments, is, however, a general property of living substance, and 

 hence demands detailed consideration. 



Among the phenomena of movement brought about by 

 contraction and expansion in accordance with the above principle 

 there can be distinguished, according to the peculiar differentiation 

 of the substratum in which they are observed, three groups, which 

 are termed : 



Amoeboid movement (protoplasmic streaming) ; 



Muscular movement (movement of smooth and cross-striated 



muscle-fibres) ; 

 Ciliary movement (movement of flagella and cilia). 



Amoeboid movement, the original form of contraction-phen- 

 omena, is found wherever there exist naked protoplasmic 

 masses, that is, cells the protoplasmic bodies of which are not sur- 

 rounded by a cell-membrane, or wherever, as in plant-cells, there 

 is within the membrane a free space for movement. As 

 examples there may be mentioned especially the manifold 

 representatives of the great protistan group Rhizoppda (Figs. 

 95 and 98); further, in the animal cell-community, leuco- 

 cytes and amoeboid wandering-cells of various kinds (Fig. 96), 

 amoeboid egg-cells of certain animals, such as sponges (Fig. 

 17, a), pigment-cells of widely different organs 1 (Fig. 97), 



1 The view often expressed in recent times, that in the movements of pigment- 

 cells there is a change of place of the granules of pigment without a simultaneous 

 change of form of the protoplasmic body, appears to me wholly untenable. 



