THE PROTOPLASMIC SYSTEM 



surface of the more heterogeneous cell-substances prevent 

 cellular disintegration. Also, are there those who unwit- 

 tingly hold a foot in each camp. 



Now, whenever there are so greatly divergent opinions, 

 one suspects either insufficient factual basis, or confusion 

 arising from too loose definition. I strongly suspect that 

 the difficulty here arises out of the fact that investigators in 

 their discussions are often not dealing with the same thing; 

 much of the argument then is beside the point. The 

 cellulose wall of a plant cell is something quite different 

 from the surface of a Paramoecium as both observation and 

 experiment show. Similarly, is it hazardous to compare 

 the surface of the red blood corpuscle in human blood with 

 that of human cells or even with the red blood cells of 

 vertebrates other than mammals since the mammalian red 

 blood corpuscle is not a true cell because it is devoid of a 

 nucleus. Therefore, if the mammalian red blood corpuscle 

 lacks a membrane this is no proof that true cells also lack 

 such. 



The first proposition is thus clear: in defining a cell- 

 membrane we must be careful to distinguish between the 

 various coverings enclosing cells, excluding, for example, 

 discussion of such formations as cellulose walls around plant 

 cells. The second proposition Is, likewise, obvious: if we 

 speak of cell-membranes, we must speak of cells; the 

 presence or absence of a membrane on a piece of a cell like 

 the mammalian red blood corpuscle does not come within 

 the discussion. The Issue concerns cells and cells only. 

 Indeed, much of the work on mammalian red blood cor- 

 puscles should be used only most judiciously In making 

 comparisons with true cells. 



Thus restricted, the discussion on the cell-membrane 

 centres around two questions: Is there a membrane around 

 animal cells ? And if present, is it a living part of the living 

 system or an exogenous formation .? If exogenous, I 



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