THE PROTOPLASMIC SYSTEM 



be so sure but doubt vanishes if I know the history of the 

 egg. Take the unfertilized egg of the sea-urchin concerning 

 the presence of whose vitelline membrane there has been 

 much dispute. Knowing the history of this egg as it 

 develops in the ovary I know that it arises by successive 

 divisions, each egg everywhere bounded by a membrane 

 except at its point of attachment to the ovaria-n wall. I 

 know also that the egg is part of the parent whose cells arose 

 by a long series of cell-divisions, of separations by cell- 

 surfaces, from a fertilized egg. This history strongly 

 indicates the presence of a membrane and the simple experi- 

 ments made previously by others which I repeat and con- 

 firm strengthen the indications. What is true of this egg is 

 true of all other eggs that I have studied: they possess 

 membranes built by the egg, living structures and not 

 adventitious depositions from the outside. In other eggs, 

 those of worms, molluscs, ascidians as well as of other 

 echinoderms, the presence of a membrane before fertilization 

 can not be doubted. The fact that membranes become 

 separated at a distance from the eggs after fertilization does 

 not mean that the eggs are now without plasma-membranes 

 for they at once form them anew. Careful observation 

 can establish this as true. 



It follows from these considerations that the membrane 

 is neither a deposition nor a precipitation induced by the 

 egg's surrounding medium. This is not to say that the 

 egg-plasma never forms precipitation-membranes. On the 

 contrary, the contents of an egg caused to burst by applica- 

 tion of pressure may often show a congealed surface. But 

 this death-change by no means constitutes evidence that 

 the membrane on the living intact egg is a precipitation- 

 membrane. A pared apple in time develops by dessication 

 an outer tough coat but this is not the same structure as the 

 removed skin. Nor does it follow on the definition of 

 protoplasm "as a film-pervaded s^'stem," which is often set 



73 



