PROOFS OF THE FOAM THEORY 233 



the same time the protoplasm bordering on the vacuole becomes 

 strongly vacuolated, and in this way the space between the 

 apparent wall of the vacuole and the external part of the proto- 

 plasmic utricle is formed. Therefore the supposed wall of 

 the vacuole may be explained as a thin layer of the proto- 

 plasm immediately surrounding the vacuole, which is forced 

 apart from the remaining protoplasm by vacuolisation. It is 

 not inconceivable that under these conditions the vacuole with 

 its surrounding protoplasmic wall may occasionally become 

 entirely isolated. In the same way the apparent wall of the 

 vacuole, as being the most internal part, may well remain living 

 the longest ; but another circumstance may also be in part 

 responsible for the non-penetration, for a long time, of the 

 eosin into the cell sap, namely, that it becomes stored up in the 

 external protoplasm. 



As has been said, I am unable to agree with either the 

 tonoplast theory or the view of the existence of a special 

 membrane proper to the vacuoles. All that can be admitted 

 in the latter respect is only that the pellicle-like limiting 

 border of protoplasm round the vacuole may possibly undergo 

 certain changes under the influence of the contents of the 

 vacuole, just as the external limit of the protoplasm under- 

 goes changes under the influence of the surrounding medium. 1 



1 With regard to this modification of the limiting layer of protoplasm 

 round the vacuole and at the external surface, i.e. the so-called " protoplasmic 

 membrane " of Pfeflfer and others, the more recent observations and views of 

 Pfeffer (1890) agree very well with my ideas. I am only doubtful whether 

 the so-called protoplasmic membrane possesses the importance for osmotic 

 processes which Pfeffer ascribes to it ; on the ground of which processes Pfeffer 

 was really first led to assume its existence ; they are now also supposed by 

 him to prove its presence, even when direct observation shows nothing of it 

 for certain. It seems to me rather that the protoplasm as such can well 

 produce these osmotic processes and their peculiarities, the more so since we 

 have found that it presents a system of the finest lamellae, the spaces of which 

 are filled with watery fluid. Pfeffer (1890, p. 238) also considers this possi- 

 bility, which I have always held to be the most probable, but arrives at the 

 conclusion "that for the protoplasmic membrane a greater resistance to per- 

 meability [i.e. than that of ordinary protoplasm] is required, for this reason, 

 that the imbibition fluid of the protoplasm apparently also contains materials 

 in solution, which do not pass out by exosmosis." Although this "appar- 

 ently," upon which Pfeffer's argument rests, does not seem to me quite incon- 

 ceivable, I yet hold to the idea that the state of the case has been essentially 

 altered by the theory of the foam-like structure of the protoplasm, which this 

 work attempts to prove. That which Pfeffer terms imbibition fluid and ap- 



