WATER 



If the cell-membrane were permeable only to water, none 

 of us could live because the cells in our bodies would receive 

 only water. The physiologist therefore has finally to postu- 

 late a membrane whose temporary break-down allows sub- 

 stances other than water to pass.^ But I think that there 

 is more to the problem than mere passage across an inert 

 boundary. The complexity of the protoplasmic organiza- 

 tion and the many reactions going on in the protoplasmic 

 system must determine, in part surely, what substances 

 coming in shall remain. What determines the ingress of 

 substances is the cell-surface. By this is not meant a 

 membrane, semi-permeable or otherwise, a dead or living 

 molecular film, but the whole ectoplasmic structure with 

 its innumerable filamentous prolongations of living active 

 cytoplasm. 



Every cell's ectoplasm is built up of such prolongations, 

 as has been abundantly shown in the chapter on the ecto- 

 plasm. The fact that this discernible, richly filamentous 

 structure exists, taken alone suffices to render untenable 



^ Bayliss, 1922, p. J12: " Js a further case of absorption, at all 

 events as it appears to me, the cell-membrane or plasma-membrane 

 may be considered. This is not to be regarded as a fixed permanent 

 structure, but as produced by deposition of cell-constituents which 

 lower surface energy at the interface between protoplasm and sur- 

 rounding medium. Thus, it changes with cell activity and is in 

 equilibrium zvith the cell contents as they alter. Thus, there is no 

 difficulty in the membrane becoming permeable in the active state 

 of the cell to substances to which it is impermeable in rest. Moreover, 

 when a fresh protoplasmic surface is produced by mechanical action, 

 a new membrane is naturally deposited on it. This is no doubt 

 why large particles can be taken up in phagocytosis through a 

 membrane which does not permit even sodium chloride in solution 

 to pass. The particles actually break the membrane, which closes 

 again behind them, in the same tvay as a needle can be passed through 

 a soap film, without bursting it, whereas a gas, such as hydrogen, 

 nearly insoluble in the soap solution, only passes zvith extreme 

 slowness." 



145 



