CHAPTER V 



THE FORMS OF CELLS 



Protoplasm, as we have already said, is a fluid* or a semi-fluid 

 substance, and we need not try to describe the particular properties 

 of the colloid or jelly-like 'substances to which it is alHed, or rather 

 the characteristics of the "colloidal state" in which it and they 

 exist; we should find it no easy matter f. Nor need we appeal to 

 precise theoretical definitions of fluidity, lest we come into a 

 debatable land. It is in the most general sense that protoplasm 

 is "fluid." As Graham said (of colloid matter in general), "its 

 softness partakes of fluidity, and enables the colloid to become a 

 vehicle for liquid diffusion, like water itself J." When we can deal 

 with protoplasm in sufficient quantity we see it /oi^§; particles 

 move freely through it, air-bubbles and liquid droplets shew round 

 or spherical within it; and we shall have much to say about other 

 phenomena manifested by its own surface, which are those especially 

 characteristic of liquids. It may encompass and contain solid 

 bodies, and it may "secrete" solid substances within or around 

 itself; and it often happens in the complex Hving organism that 

 these sohd substances, such as shell or nail or horn or feather, 

 remain when the protoplasm which formed them is dead and gone. 

 But the protoplasm itself is fluid or semi-fluid, and permits of free 

 (though not necessarily rapid) diffusion and easy convection of 

 particles within itself, which simple fact is of elementary importance 



* Cf. W. Kuhne, Ueber das Protoplasma, 1864. 



t Sand, or a heap of millet-seed, may in a sense be deemed a "fluid," and such 

 the learned Father Boscovich held them to be {Theoria, p. 427), but at best they 

 are fluids without a surface. Galileo had drawn the same comparison; but went on 

 to contrast the continuity, or infinite subdivision, of a fluid with the finite, dis- 

 continuous subdivision of a fine powder. Cf. Boyer, Concepts of the Calculus, 1939, 

 p. 291. . 



J Phil Trans, clt, p 183, 1861; Researches, ed. Angus Smith, 1877, p 553. 

 We no longer speak, however, of "colloids' in a specific sense, as Graham did; 

 for any substance can be brought into the "colloidal state" by appropriate means 

 or in an appropriate medmm. 



§ The copious protoplasm of a Myxomycete has been passed unharmed through 

 filter-paper with a pore-size of about 1 /x, or 0001 mm. 



