STRUCTURE OF PROTOPLASM 45 



appears to be constant and comparatively unchangeable, merely because 

 the elements of which it is built up are invisible to us. Indeed, even 

 if we could see them, that would help us as little towards understanding 

 the vital mechanism, as our being able to distinguish the legs of a marching 

 army would enable us to find out the causes and reasons why the legs 

 are set in motion. 



It is an absolute necessity that there should be only a slight degree of 

 cohesion between its component units, in order to permit of those per- 

 petual changes and alterations which the continued life of the protoplast 

 necessitates. The plasma as a' whole is viscous or semi-fluid. At the same 

 time it is quite possible that marked cohesion may exist temporarily or 

 even permanently between separate elements. Indeed, the protoplasm is able 

 to increase its own powers of cohesion in fern antherozooids, in the cilia 

 of zoospores, and in the tough external periplasm of Euglenae. In the 

 peripheral layers of the plasmodia of myxomycetes, changes of consistency 

 may be observed similar to those which take place in gelatine when 

 alternately warmed and cooled. Such changes probably play an important 

 part in the vital mechanism, at one time a particular unit or molecule 

 of albumin being in a fluid, at another in a solid condition 1 . Pfaundler 2 , 

 indeed, explains the peculiar properties of viscous solids (sealing-wax, &c.) 

 by assuming the occurrence in them of molecular changes of some such 

 character. 



The chromosomes, and many other bodies found in the cell, are 

 probably of a more or less gelatinous consistency, but it is difficult to say 

 whether they approach more to the character of fluids than of solids, or 

 vice versa. Owing to the extremely small size of all such bodies, the 

 influence exerted by surface tensions in maintaining their shape is relatively 

 extremely great. Thus a tiny air bubble adhering to the side of a glass 

 vessel filled with water exhibits several more or less marked characteristics 

 of a solid owing to its relatively high surface tension. Similarly, a fine 

 emulsion of any two non-miscible fluids is more viscous and less fluid 

 than either of the two liquids of which it is composed. 



The semi-fluid nature of the protoplasm results then in a tendency 

 to assume a shape, or position of equilibrium, determined by similar 

 mechanical properties to those which give a drop of water its tendency to 

 become spherical. When living, this property may be more or less markedly 

 counteracted by other influences. The changes of shape shown by living 

 protoplasm are signs of continuous internal changes, which may often be 

 so short in duration as to produce no perceptible result, while before 



1 For details, see Pfeffer, Zur Kenntniss d. Plasmahaut u. Vacuolen, 1890, p. 253; Biitschli, 

 Unters. uber mikroskop. Schaume, 1892, pp. 144-171 ; Hertwig, Zelle, 1893. 



2 Pfaundler, Sitzungsb. d. Wiener Akad., 1876, Bd. LXXIII. Abth. 2. p. 253. 



