500 



THE CELL AND MAMMALIAN HISTOLOGY 



of dead cytoplasm induced by the action of the fixatives ; a 

 solution of protein, such as white of egg, when treated in the same 

 way shows the same structure, although it previously had none. 

 All modern work shows that most cytoplasm is, in any ordinary 

 sense of the word, structureless, so that there can be free move- 

 ment from one side of the cell to the other. It may, however, 

 have what might be called potential structure, which is realised 

 on death, and this is presumably based on chemical differentiation 



HLjdrocarbon 



groups 



•Adsorbed probein 



Fig. 391. — Diagram of molecular structure of the plasma membrane. — From 

 Danielli. Bourne, Cytology and Cell Physiology, 2nd edition, 195 1. Clarendon 

 Press, Oxford. 



within the cell. The obvious ability of cytoplasm to flow, as in 

 Amceba or in many plant cells, shows that it is a liquid, and many 

 determinations of its viscosity (that is, its internal friction, or 

 resistance to flow) show this to be only a little greater than that of 

 water. At times, however, some cytoplasm is much stiffer, and 

 must be accounted as solid, as in the outer part of an Amceba. 

 The simple explanation is that all cytoplasm is a colloidal solution, 

 comparable to that of gelatine and water ; usually water is the 

 continuous phase, and the material is liquid and flows, but at times 

 the proteins make the continuous phase and the material gelates 

 and is solid. Reversible changes of phase are easily induced in 



