CELLS AND TISSUES 5 



Living and Fixed Cells. — The majority of cytological descriptions 

 have been based on material which has been prepared for observation by 

 complicated processes involving (1) fixation, whereby the cell structures 

 are rendered firm and capable of enduring the subsequent treatments 

 without undue distortion, (2) dehydration, (3) imbedding in paraffin, (4) 

 sectioning into thin slices, (5) staining with various dyes to bring out 

 contrasts, and (6) mounting in a medium, usually Canada balsam, having 

 a refractive index equal to that of the slide and cover-glass between which 

 the material is supported. Consequently cytologists have had to meet 

 the criticism that they are studying a series of artificial appearances 

 rather than the natural structures of cells. It is well known that the 





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Fig. 2. — Root-tip cells stained with iron alum-htematoxylin after fixation in various 

 aqueous solutions: o, ammonium and potassium bichromates, copper sulphate and pyridine; 

 b, chromic anhydride, butyric acid, and nickel hydroxide; c, formic acid, chromic anhydride^ 

 and acetaldehyde; d, trichloracetic acid, formaldehyde, and nickel hydroxide, o, b, d 

 are from Zea; c from Allivm. Fluids with a pH below about 4.2 to 4.6 tend to give the " acid 

 fixation image" (c) desired in studies of chromosomes, while those with a higher pH tend 

 to give the "basic fixation image" (d) showing the chondriosomes. {Photographs con- 

 tributed by C. Zirkle.) 



coagulative action of a fixing fluid may produce a visible structure not 

 previously present in the protoplasm, and that some fluids swell certain 

 cell components while others shrink them. Moreover, it is known that a 

 given fluid may preserve the nucleus well and the cytoplasm poorly, or 

 vice versa, and that the fixation images may differ characteristically 

 according to the pH and other chemical features of the fluids used (Fig. 

 2). 2 Hence careful cytologists are attempting to check their observations 

 on fixed and stained cells against living material as far as possible and are 

 constantly devising and refining methods for the study of untreated 

 protoplasm. 



Nearly all of the cell components mentioned on the foregoing pages 

 may be observed to good advantage in living cells. For such study 

 various kinds of cells can be kept aUve for long periods if mounted in 



2 See Yamaha (1926 et seq.) and Zirkle (1928a6, 1929a). 



