154 L - v - HEILBRUNN. 



leads not to transformation of gel to sol, as we would have to sup- 

 pose, but on the contrary such an influx transforms the egg pro- 

 toplasm from the sol to the gel condition. The experimental 

 evidence in support of this fact is given on p. 195. Water absorp- 

 tion by the egg is indeed correlated with a loss of water on the part of 

 the egg proteins. The passage of water into and out of the cell 

 can not therefore be due to changes in the water content or 

 aggregation state of the egg proteins, and the first possibility is 

 unable to account for the facts. Only the second possibility 

 remains, i. e., that the vitelline membrane acts as a semiper- 

 meable membrane and controls osmotic intercourse. On the 

 basis of this view many facts are understandable, which can be 

 explained in no other way. This will, I think, be demonstrated 

 in the course of the argument. 



The plasma-membrane of a cell is defined as the membrane 

 which governs its osmotic intercourse. According to this defi- 

 nition, the vitelline membrane is the plasma-membrane of the 

 Arbacia egg cell. Hitherto no one has either described a plasma 

 membrane, or studied directly the properties of one. Although 

 he often uses the concept of such a membrane, Lepeschkin ('n) 

 admits that the actual structure is "zurzeit unbekannt." Loewe, 

 ('13) in referring to the plasma-membrane, says: '"Allein weit 

 davon entfernt. dasz auch nur ihre Existenz irgendwie sicher- 

 gestellt ware, sind auch iiber die Beschaffenheit dieses hy- 

 pothetischen Gebildes die Meinungen so zahlreich wie die 

 Moglichkeiten." Fischer ('12) is indeed of the opinion that 

 "the entire conception of an osmotic membrane about cells is an 

 impossibility." 



The plasma-membrane of the Arbacia egg is a protein gel. 

 As such, it possesses a certain degree of rigidity. Suppose a 

 hypothetical system completely surrounded by an extremely 

 rigid semipermeable membrane. 1 If such a system were placed 

 in a concentrated solution no exosmosis could take place, for if 

 the membrane were perfectly rigid, there could be no removal 

 of solvent from the system without the production of a vacuum. 

 But the membrane would be subjected to a considerable pressure, 

 which would tend to make it rearrange its particles in such a 



1 Possibly this is the case in the Fundulus egg. 



