228 PROTOPLASM 



protoplasm have, however, rather different views with regard 

 to the limitation of the vacuoles. In the first place, none of 

 them have really given any account of why the vacuoles 

 should always be spherical, although the rigidity of their 

 framework was yet necessarily admitted. In general the 

 structures in question are regarded as larger collections of 

 fluid in certain regions of the framework, which is thereby 

 thrust apart. These views only vary on the point as to 

 whether the contents of the vacuoles are to be considered 

 as identical with the general contents of the matrix of the 

 alveolar framework, or as differing from it. Thus Heitzmann 

 has termed the vacuole " a lake in the middle of the proto- 

 plasmic body" (1883). Now, if the framework is rigid, it 

 is not at all apparent why the vacuole when unrestrained 

 should always possess a spherical, and not a more or less 

 irregular outline. But it remains still more inconceivable 



o 



how the adherents of the framework theory can speak of a 

 flowing together of vacuoles, since this is simply impossible. 



If the vacuole is only a collection of fluid in certain regions of 

 the framework, there must exist, originally at any rate, a connec- 

 tion between the contents of the vacuole and the intervening 

 matrix. That such a connection, however, cannot be permanent 

 is proved not only by direct observation, which always shows a 

 distinct and continuous limiting margin to the vacuole, but also 

 by the common experience, that the cell sap cavity of vegetable 

 cells, which is in reality nothing more than a very large vacuole, 

 frequently contains a coloured fluid, while the enchylema of the 

 protoplasm is completely uncoloured. This, as well as other 

 observations, make it unconditionally necessary that the contents 

 of the vacuole should be shut off from the enchylema by a closed 

 lamella. In this way Schmitz (1881) had already assumed that 

 the origin of the vacuoles was due to the formation of " a special 

 continuous limiting layer " round the cavities of the framework. 

 But this would only be possible, if the framework contracts, 

 by closing its meshes to form a continuous membrane round 

 the vacuole. Van Beneden also arrived, in 1883, at similar 

 views. If, he declares, the contents of the vacuole is identical 

 with the inter-fibrillar substance, the wall of the vacuole must 

 possess a lattice-like structure with extremely narrow meshes. 

 He therefore imagines in any case that this wall arises by con- 

 traction of the framework. Leydig (1883), who derives the 



