THE BIOLOGY OF THE CELL SURFACE 



presume that It is non-living. I think that the question of 

 necessity or purpose is not involved. To say that one does 

 not see how it is possible for a cell to remain intact without a 

 membrane most certainly does not mean that it can not thus 

 remain. Here I speak only of those cells that I know best, 

 egg-cells, though I hold that the discussion can apply to 

 some extent also to other animal cells which, like the eggs, 

 are often held to be without membranes. 



In the description given above I pointed out that on eggs, 

 as, for example, that of Platynereis, two membranes are 

 present; the outer or vitelline, and the inner or plasma- 

 membrane. Whenever egg-membranes are discussed, the 

 two membranes should be clearly distinguished. Further- 

 more, when the vitelline membrane is spoken of, it should be 

 clearly stated whether the unfertilized or the fertilized egg 

 is considered. Much of the disagreement concerning the 

 nature and properties of egg-membranes is due to the 

 confusing of the vitelline and the plasma membrane and to 

 failure to appreciate the difference of the former before and 

 after fertilization. 



Generally, the essential difference between a vitelline and 

 a plasma-membrane lies in the fact that the latter is con- 

 tinuous with the egg-cytoplasm whilst the former is dis- 

 continuous with it, set apart from it and separated by a 

 space, the so-called perivitelline space. Using this simple 

 distinction we meet, however, with the difficulty that in 

 many eggs that membrane which after fertilization is thus 

 so clearly set off as a vitelline membrane, before fertilization 

 appears as a plasma-membrane, continuous with the cyto- 

 plasm. When after fertilization such a vitelline membrane 

 separates from the egg, on the cytoplasmic surface of the 

 egg appears a new plasma-membrane. 



In observing one living egg I may be able to say in a 

 moment that a membrane is present, it being so sharply 

 differentiated; observing another species of egg, I may not 



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