FORMATION OF MEMBRANES 109 



something made by the cell and deposited outside like the crus- 

 tacean shell. It is just as much part of the cell as the protoplasm 

 itself. Unless this point is understood, difficulties will be found 

 in the study of alterations in permeability. The animal cell 

 membrane must be considered as a part of the cell, having a 

 similar metabolism to the interior of the cell and dying when the 

 rest of the cell dies. 



The exact chemical composition of animal cell membranes 

 is not known, but modern research tends to show that it is similar 

 to that of the cell as a whole. We are meantime more concerned 

 with their formation and structure. The fourth essential quality 

 of a cell membrane mentioned above gives a clue as to the mode 

 of its origin. 



Formation. 



The only membrane that could answer to this test, i.e. as 

 capable of instantaneous formation and expansion, is one formed 

 by a Gibbs-Thomson deposition of solutes on the surfaces (see p. 48). 

 That such a membrane can be formed is readily demonstrable. 



1. Brailsford Robertson's artificial amoeba (Part II. p. 403) 

 shows mobility and keeps intact for some time. 



2. Egg albumin solution forms a pellicle or coat of great tough- 

 ness, cf. meringues. 



3. Traube's membranes, especially in the hands of Leduc (Part 

 II. p. 399), yield life-like growths. 



Strictly speaking, a substance which is adsorbed under certain 

 circumstances will be set free when the circumstances are reversed. 

 Many substances, however, undergo alteration in physical state 

 on adsorption. For example, in the formation of meringues, the 

 egg white becomes coagulated and so becomes incapable of re- 

 entering the liquid state. Such an irreversible reaction is termed 

 pseudo- adsorption. 



Adsorption (including pseudo-adsorption) is, as we have seen 

 in the last chapter, dependent on the ability of the adsorbed 

 substance to lower surface tension. Now, from its very nature, 

 surface tension has a negative temperature coefficient. Increase 

 of temperature lowers surface tension. It follows that increase 

 of temperature will diminish the amount of material adsorbed, 

 and, conversely, a decrease in temperature will increase the ad- 

 sorbability of substances in solution. Can we associate with this 

 fact the varying thickness of membranes according to their 

 degree of exposure to cold ? 



