CHAPTER V 



THE FORMS OF CELLS 



Protoplasm, as we have already said, is a fluid or rather a 

 semifluid substance, and we need not pause here to attempt to 

 describe the particular properties of the semifluid, colloid, or 

 jelly-like substances to which it is allied; we should find it no 

 easy matter. Nor need we appeal to precise theoretical definitions 

 of fluidity, lest we come into a debateable land. It is in the most 

 general sense that protoplasm is "fluid." As Graham said (of 

 colloid matter in general), "its softness partaJces of fluidity, and 

 enables the colloid to become a vehicle for liquid diffusion, like 

 water itself*." When we can deal with protoplasm in sufficient 

 quantity we see it flow ; particles move freely through it, air- 

 bubbles and hquid 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" within or around itself solid substances; and very 

 often in the complex living organism these solid substances 

 formed by the living protoplasm, like shell or nail or horn or 

 feather, may remain when the protoplasm which formed them 

 is dead and gone ; but the protoplasm itself is fluid or semifluid, 

 and accordingly permits of free (though not necessarily rapid) 

 diffusion and easy convection of particles within itself. This simple 

 fact is of elementary importance in connection with form, and 

 with what appear at first sight to be common characteristics or 

 pecuharities of the forms of living things. 



The older naturalists, in discussing the differences between 

 inorganic and organic bodies, laid stress upon the fact or state- 

 ment that the former grow by "agglutination," and the latter by 



* Phil. Trans, cli, p. 183, 1861; Researches, ed. Angus Smith, 1877, p. 553. 



