THE BIOLOGY OF THE CELL SURFACE 



not others. Much of the work on which the theory of 

 semi-permeability is based is derived from work on artificial 

 inanimate membranes. Many investigators regard the 

 cell membrane as a molecular film; others treat it as such 

 in their physico-chemical and mathematical disquisitions. 

 The chemical nature of the postulated semi-permeable 

 membrane around cells has often been discussed. Depend- 

 ing upon the one or another theory, it is considered protein, 

 lipin, a mosaic of lipin and protein or a more complicated 

 chemical structure. As was pointed out, to some the mem- 

 brane is a dead, inert thing, a precipitation out of the proto- 

 plasm, or merely a surface conditioned by the external 

 medium, etc. For others the membrane is a living part 

 of the cell-structure. 



In the above given descriptions of eggs, a vitelline mem- 

 brane has been spoken of. This is built by the egg as it 

 develops to maturity and is of measurable width. Beneath 

 this vitelline membrane can be seen in many eggs a very 

 delicate structure, appearing in optical section as an 

 extremely thin line, the plasma-membrane, also built by 

 the egg; it is visible and of measurable thickness and not a 

 molecular film. The surface-structure of the cell is, how^ 

 ever, not to be thought of as comprising only this plasma^ 

 membrane. The deformation of the water drops as they 

 leave the egg takes place in the surface-structure below the 

 plasma-membrane. This cell-structure is the ectoplasm. 

 On these eggs as on so many others it displays amoeboid 

 activity. Changes in a plasma-membrane and certainly 

 those in a molecular film can not account for the rhythmical 

 difference in permeability so often described and postu- 

 lated,^ for which, however, the ectoplasm, both in width 

 and in activity, offers a sufficient basis. 



1 For examples of rhythmical changes, see Bayliss, /p/J. 



140 



