PERMEABILITY OF ERYTHROCYTES. 37 



means of concentrated or 30 per cent, solution of potassic hydrate, or 

 with some preservative fluid, without friction. By softening them with 

 the aid of concentrated tartaric-acid solution the leukocytes appear 

 with especial distinctness. Often, however, search for the presence of 

 blood-corpuscles will be fruitless. Red, suspicious fluids are examined 

 directly. If the blood-corpuscles in the fluid have possibly already 

 become pale, or if they are present only as stroma, the addition of a 

 wine-yellow aqueous solution of iodin-potassium-iodid to the micro- 

 scopic preparation will at times render them much more distinct. 

 Saturated solution of picric acid, 20 per cent, solution of pyrogallic 

 acid and 30 per cent, solution of silver nitrate have also been recom- 

 mended for this purpose. 



PERMEABILITY OF ERYTHROCYTES. ISOTONIA (HYPERISO- 

 TONIA AND HYPISOTONIA). DEMONSTRATION OF THE 

 STROMA-LAKE COLORATION OF THE BLOOD. 



All substances soluble in water attract water with a certain intensity. 

 The energy by means of which this attraction takes place is known as 

 hygroscopic energy or osmotic tension. The manner in which this behaves 

 with regard to living cells was discovered by de Vries (1884). A 

 vegetable cell consists of a membrane, which is permeable to salts and 

 to water. This membrane is in contact by its inner surface with the 

 adjacent cell-protoplasm, which likewise is permeable to water, but not 

 to salts. If fresh vegetable cells are placed in distilled water, this 

 passes through the cell-membrane and through the cell-protoplasm, 

 and causes the cells to swell. If, however, the cells are placed in a strong 

 saline solution, the cell-contents shrink, because water is abstracted 

 from them. The shrinking of the cellular protoplasm is shown by the 

 fact that the protoplasm contracts upon all sides and becomes detached 

 from the cell-membrane. This detachment of the shrunken cell-body 

 from the cell-wall in consequence of loss of water is designated plasmolysis 

 by de Vries. 



Plasmolysis is the more pronounced the more concentrated the 

 saline solution surrounding the vegetable cell. The saline concentra- 

 tion that brings about the first signs of plasmolysis can be determined 

 experimentally for every variety of cell. The different salts must be 

 employed in various concentrations, in order to bring about the same 

 degree of plasmolysis. Solutions of different salts that exert the same 

 effects in the process of plasmolysis are designated isotonic solutions. 

 The necessary concentrations are to each other as the molecular weights 

 of the different salts. For instance, a 0.58 per cent, solution of sodium 

 chlorid causes the beginning of plasmolysis in the same way as a i.oi 

 per cent, solution of potassium nitrate, or as a 1.5 per cent, solution 

 of sodium iodid. The molecular weights of the three substances are 

 58, 10 1 , and 150 respectively. Isotonic solutions have the same freezing- 

 point, which always becomes lower with increasing concentration; and 

 also the same boiling-point, which becomes higher with the degree of 

 concentration. 



There is thus for the red blood-corpuscles a given concentration for 

 certain but not all substances in which they neither shrink nor swell. 

 For mammalian erythrocytes this is a 0.9 per cent, solution of sodium 

 chlorid for the frog 0.6 per cent. If the equally effective degree of 



