134 MEMBRANES {PLASM AH AIJT) 



of expansion. Mere elasticity will not answer this end. 



The membrane must be capal^le of almost instantaneous 



growth. 

 The animal cell membrane must not be considered as a box 

 or container in which the cell protoplasm has been placed. It is 

 not something apart from the cell like an eggshell. It is not even 

 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. A very simple experiment shows the intimate 

 relationship of cytoplasm and membrane. Amoebae immersed in 

 a dilute aqueous solution of eosin remain unstained as long as they 

 are alive. That is, the membrane is impermeable to the dye. 

 Eosin, on the other hand, injected into the body of the amoeba 

 remains evenly distributed throughout the protoplasm during life, 

 and does not diffuse into the surrounding water. In both experi- 

 ments the cell wall becomes permeable to the dye on the death of 

 the animal. 



Formation. 



The fourth essential quality of a cell membrane mentioned above 

 gives a clue as to the mode of its origin. 



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 on the surfaces. Any substance 

 in the protoplasm which lowers surface tension will have a greater 

 concentration at the surface than in the interior. If there is more 

 than one capillary active substance present, then the one in 

 possession of the surface will be the substance having the greatest 

 power of lowering surface tension. Now a surface implies the 

 juxtaposition of two systems or phases, e.g. amoeba-water, 

 erythrocyte-plasma, cell-environment. The second system, the 

 environment, must, therefore, contribute its quota to the formation 

 of the membranes. That such a membrane can be formed is 

 readily demonstrable. 



1. Brailsford Robertson's artificial amoeba (Part II., p. 516) 

 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. 544), yield life-like growths. 



