34 THE CELL 



so far as this property has yet been studied, are shown to be permeable to 

 chlorine, sulphuric acid, carbon dioxide, iodine, bromine, oxalic acid, phosphoric 

 acid, salicylic acid, benzoic acid, and arsenic acid (Hamburger and v. der 

 Schroeff). 



Overton has studied the permeability of cross-striated muscle to a large 

 number of organic compounds, and has come to the conclusion that toward the 

 same substances they behave just as do plant cells. All compounds which are 

 plainly soluble in water and are also soluble in ethyl-ether, in the higher alco- 

 hols, in olive oil and similar organic solvents, or which at least are not much 

 more difficultly soluble in these than in water, penetrate living muscle fibers and 

 other animal and plant cells very easily. But the more the solubility of a com- 

 pound in water exceeds its solubility in one of the organic solvents, the more 

 slowly does it penetrate these structures. For explanation of this peculiar 

 behavior, Overton has put forward the hypothesis that the limiting layers of 

 the protoplasm are impregnated with a fatty substance, a mixture of lecithin 

 and cholesterin, and that the elective solvent power of this mixture for definite 

 substances governs the pure osmotic permeability of the cells. 



Out of some 75,000 organic compounds known at present more than 60,000, 

 to accept Overton's rough estimate, can penetrate the cell. Among these how- 

 ever are found neither the carbohydrates nor a number of other substances par- 

 ticipating actively in the metabolism of plants and animals. Overton remarks 

 that so far as the constitution of these compounds is known, derivatives of them 

 can always be found which do penetrate the cells very easily. How far his 

 explanation applies to the living body we do not know. 



It has been found that different Infusoria are to a great extent independent 

 of the osmotic pressure of the solution, since they can exist for days at a time in 

 distilled water, without suffering noticeably and without exhibiting any very 

 striking changes in form (Goldberger). The eggs of Fundulus also do not 

 swell if they are suddenly brought from sea water into distilled water ; and they 

 do not shrink if the reverse change is made (Loeb). McCallum found in the 

 case of the medusa, Aurelia, that the salt content of the surrounding medium 

 can vary within wide limits without materially affecting the body fluid. The 

 fluid pressed out of the body contains less SO 3 , MgO, and Na 2 O, and more 01, 

 Fe, and especially K than the sea water; the depression of the freezing point 

 also was less for the latter than for the body fluid. While the salt content of 

 the blood of the green crab increases and decreases with that of the sea water, 

 the relation in the crayfish is just the reverse : here the depression of the freezing 

 point of the blood is 0.8 C., while in the surrounding water it is only 0.02- 

 0.03 C. (Fredericq). Frogs kept for weeks in distilled water give up only a 

 part of their salt to the water, notwithstanding that between them and the water 

 there is a difference of osmotic pressure amounting to about two atmospheres 

 (Durig). 



In the different tissues of any given animal also there are noteworthy differ- 

 ences with respect to osmosis. Membranes consisting of only a single layer of 

 connective tissue coated with a single layer of smooth muscle cells, like the 

 peritoneum and the mesentery, offer very little resistance to the passage of dif- 

 ferent ions ; and their permeability is not noticeably changed after the death of 

 the cells by chloroform. But living membranes constructed of specifically dif- 

 ferentiated epithelial cells behave differently: their ability to oppose or to facili- 

 tate the passage of ions corresponds to the physiological functions devolving 

 upon them, and disappears with the death of the cell, at which time also the 

 permeability rises significantly (Galeotti). 



