THE PLASM ATIC MEMBRANES 107 



concluded that permeability may also be modified by the conditions under which 

 the membrane is formed. 



Precipitation-membranes of tannic acid with gelatine or mucilage grow and 

 increase in size with perfect regularity, provided no disturbing influences are 

 present. When the solution of tannin is replaced by water, the conditions 

 necessary for growth are removed, so that, as the osmotic pressure continues to 

 increase, the delicate separating membrane soon ruptures. The surface growth 

 of membranes formed by the precipitation of ferrocyanide of copper or prussian- 

 blue can only continue for a short time, and to a limited extent. As soon as 

 further growth ceases, the increasing internal osmotic pressure soon ruptures the 

 cell. The fluid, which then exudes, becomes immediately invested by a new 

 precipitation-membrane, and the repetition of this process may result in the 

 production of the most curious figures \ 



SECTION 18. The Plasmatic Membranes. 



The general principles previously laid down as governing the exchanges 

 of substance taking place between the protoplast and the external world 

 would still hold good, even though no differences existed between the 

 diosmotic properties of the general mass of the protoplast and the limiting 

 membranes which enclose it ; if, for example, the entire thickness of the 

 primordial utricle behaved like a single very thick plasmatic membrane. 

 There are, however, weighty reasons for concluding that the deductions 

 arrived at in Sect. 16 are correct ; but only a short account of these reasons 

 can be given here 2 . 



The question at issue would be definitely answered in the manner 

 already indicated, if it were found that substances, commonly present 

 or artificially introduced, diffused through the central mass of the plasma, 

 but did not appear in the vacuoles or in the water outside, proving that 

 they are incapable of diosmosing through the plasmatic membranes. As 

 a matter of fact, a variety of observations make it probable that such is 

 actually the case in the living protoplast, but no absolutely convincing 

 proof has as yet been brought forward. When, however, the plasmatic 

 membrane is caused to assume a condition of rigor by treatment with 

 very dilute hydrochloric acid, its original diosmotic properties are usually 

 at first retained, and a dye, which is unable to penetrate the plasmatic 

 membrane, will, if it finds entry through a slit in the latter, rapidly diffuse 

 through the enclosed central mass of the dead plasma. 



1 The objections which Sachs (Lehrb., 4. Aufl., 1887, p. 645) raises against Traube are based 

 on the production of these eruptions. In the production of the so-called myelin forms, similar 

 eruptive actions probably also take part. See Briicke, Sitzungsb. d. Wien. Akad., 1879, Bd. LXXIX, 

 Abth. iii, April. 



3 Details by Pfeffer, Ztir Kennt. d. Plasmahaut u. d. Vacuolen, 1890, pp. 224, 244. For an 

 account of the nomenclature employed, see the same work, p. 188, and also Sect. 8 of this book. 



