12 



PRINCIPLES OF GENERAL PHYSIOLOGY 



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It is difficult to see what purely chemical relationship can exist between complex, 

 substituted diazo-sulphonates, as a large number of these specific dyes are, 

 and the chemical constituents of cells. Moreover, although methylene blue and 

 other thiazines are specific vital stains for nerve tissue, certain safranin azo dyes 

 diazingreen, for example which have no chemical relationship to the former, 

 are also vital nerve stains, while similar compounds of the same safranin series 

 have no such property (Michaelis, 1902, p. 104). At the same time, one must 

 not be too dogmatic where so little is definitely known. It will be necessary 

 to discuss this question further in later chapters, and we shall see also that the 

 conception of giant molecules in the chemical sense has very little evidence in 

 its favour. For the present, it suffices to point out the fact that this specific 



affinity of dyes to particular structures exists, 

 whatever may be its explanation. . 



Much caution must be exercised in the 

 interpretation of the results obtained by injec- 

 tion of dyes into living organisms. Very 

 few are entirely devoid of poisonous properties, 

 some are very toxic, so that when any one of 

 these is found within a cell we have no means of 

 knowing with certainty whether it found its way 

 there while the cell was still alive, or whether it 

 killed the cell first and then subsequently found 

 its way to the inside, unless we have some 

 criterion as to the vitality of the cell at the time 

 when it is examined. This is to a certain degree 

 possible in the case of unicellular motile organ- 

 isms, but in the case of the tissues of the higher 

 organisms the difficulty is obviously greater. 

 Evidence may be obtained by the investigation 

 of the permeability of the cell with respect to 

 innocuous bodies, by methods to be referred to 

 in Chapter V. It is clear that a dye cannot 

 stain any constituent of a cell if unable to pass 

 through the covering membrane, but it is not 

 always possible to be certain that, when it does 

 pass through, this happens without previously 

 producing changes in the membrane itself. The 

 dye may also be able to pass through the nonnal 

 membrane, but may kill the cell when it reaches 

 the internal structures. Statements are some- 

 times made with regard to the permeability of 

 cells to dyes without taking due account of these 

 possibilities. Many dyes, such as methylene blue, 

 are reduced to colourless derivatives by certain 

 cells while alive, but not when dead, so that 

 reducing power in these cases may be used as a criterion of vitality (Michaelis, 

 1902, pp. 101 and 104). The living nucleus appears to be unstainable, so that 

 when we see it begin to take up pigment we have warning of the death of the cell. 

 Neutral red is one of the least toxic of intra-vital stains. 



Fixed cells behave to dyes quite differently from living cells; recently dead, but unfixed, 

 cells have also properties in this respect unlike both those of fixed cells and those of living 

 cells. It is probable that valuable information might be obtained from more detailed study 

 of changes in dying protoplasm. 



A portrait of Ehrlich will be found in Fig. 11. 



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K. 



FIG. 12. DIAGRAM OF PART OF 

 A CEJ.L or SPIROGYRA. 



0, reticular coagulation of cell sap, 

 as occurs after action of osmic acid. 

 A', paths of the "dancing" particlt-.s 

 (Brow-Mian movement), as seen in 

 the living- cell ; these paths are 

 much longer than the meshes of 

 the reticulum, so that the latter 

 could not be present in life and 

 must be a product of the action 

 of the fixative. 



(After Flemming.) 



FIXATION 



Although protoplasm shows so little structure in the living state, it might 

 be thought that, by use of fixing and staining reagents, more could be made 



