STUDIES IN ARTIFICIAL PARTHKM >< ,|..\ KM-,. 169 



with it and the entire egg increases in diameter. According 

 to this view cytolysis (when not directly an osmotic phenomenon) 

 is in every case due to a lowering of the surface tension of the 

 plasma membrane. This produces an influx of water into the 

 cell. 



Various other explanations of cytolysis have been attempted. It has been 

 regarded as due to the swelling of the egg proteins. This is an incorrect assump- 

 tion, as I have already pointed out that the egg proteins, instead of becoming 

 swollen, are coagulated in the cytolyzed egg (see pp. 195-198). It might also be 

 considered as the result of a change in the plasma-membrane, of such a sort that 

 this membrane would become readily permeable for salts though still impermeable 

 for colloids. The unopposed osmotic pressure of the egg colloids would then push 

 out the plasma membrane and the egg would increase its diameter. But such an 

 explanation can not hold generally, for membrane-swelling although it produces 

 cytolysis, does not render the plasma-membrane readily permeable to salts. In- 

 deed in hypertonic salt solutions, eggs with swollen membranes shrink even more 

 than those with unswollen membranes (see pp. 155-158). This is sufficient proof 

 that the swollen membrane retains its semipermeability. 



In an earlier paper it was pointed out that all substances 

 which had been described as producing membrane elevation 

 (as well as some other newly discovered ones) did actually lower 

 surface tension in aqueous solution. In a few substances, how- 

 evei, this lowering could apparently only be a very slight one. 

 These cases have been reinvestigated, and they have been found 

 to involve membrane swelling rather than membrane elevation. 



When membrane swelling is rapid, it is possible that the lowered 

 surface tension so produced should be followed by membrane 

 elevation if the eggs are immediately returned to sea-water. 

 One such case was indeed observed, but the observation was 

 made at the close of the season, and further study of this point 

 is necessary. 



D. Cortical Action of Heat and of Various Chemicals. 



Heat. Although heat has been described as producing arti- 

 ficial parthenogenesis in the sea-urchin egg (McClendon, ! io), 

 I know of no description of membrane elevation as a result of 

 heat treatment. In several experiments, I was unsuccessful 

 in producing membrane elevation in this fashion. In experiment 

 a, eggs were dropped into sea-water which had been heated to 

 36.5 degrees, and were exposed I, 2, 3, 4 minutes. In experi- 

 ment b, the eggs were dropped into sea-w r ater heated to 35 de- 



