VACUOLES AND THE FOAM THEORY 229 



vacuoles from " enlargement and flowing together " of the cavities 

 of the meshes of the framework, seems to think that they are 

 usually not definitely shut off from the enchylema. At least he 

 remarks that they seldom obtain a definite lining by condensa- 

 tion of the trabecular framework (p. 143). Finally Heitzmann 

 also (1883) developed similar views. According to him the 

 vacuoles always possess a special wall, which is built up in the 

 form of a so-called " membranous layer." By such a membranous 

 layer he understands a delicate closed membrane, which is thus 

 formed. From the nodal points of the network, which as a rule 

 only send out a few filaments of the meshwork, such a number 

 of threads radiate out in one plane that they fuse into a con- 

 tinuous layer, while the nodal points at the same time disappear. 

 Now, since the membranous layers formed by neighbouring 

 nodal points become united into a continuous layer, there arises 

 a continuous envelope round the vacuole. Frommann also states 

 ,(1890, p. 10) that the vacuoles possess "for the greater part a 

 .delicate pale or somewhat shiny membrane." He even seems 

 to regard this membrane as solid, since he remarks that the 

 growth of the vacuoles " takes place by fusion of neighbouring 

 vacuoles, accompanied by a tearing or liquefaction of the mem- 

 brane," or by osmosis. 



Finally C. Schneider (1891), whose views upon the fibrillar 

 structure of protoplasm we described above (p. 182), has also 

 discussed the formation of the vacuolar wall. He believes that 

 he is able to find his fibrillae even in the vacuolar wall. In this 

 place they are united by a special cementing substance into a 

 membrane. What Schneider regards as the fibril!^ in the wall" 

 of the vacuole is certainly nothing else than a surface view of 

 the layer of alveoli which borders on the vacuole. 



We have seen in the foregoing to what assumptions the 

 representatives of the framework theory are forced in order 

 to explain the closed vacuolar wall. If we further imagine 

 to ourselves the apparatus of mysterious forces which bring 

 about the condensation, or even the closure, of the meshes 

 of the framework round the vacuoles, and on the other 

 hand reproduce the usual condition of the network after 

 the disappearance of the vacuole, it would seem clear that 

 the correct explanation will be attained with some difficulty 

 on this theory. 



On the other hand, the alveolar, or foam theory of 

 protoplasm explains the actual phenomenon, and the be- 



