108 MICROSCOPICAL TECHNIQUE. 



linear crack ; but this will be larger in the centre of the grain than 

 towards the edges, just the reverse of what occurs in wheat. 



Iron-Haematoxylin and Centrosomes.'^— Iron-haematoxylin has 

 been used by Heidenhain in the study of the centrosomes and 

 astrospheres. The original process, which is also repeated in the 

 new modification, was the following : — 



Fine sections of preparations in sublimate are fixed on the 

 slide by means of distilled water, dehydrated with alcohol con- 

 taining iodine, and exposed to a \\ per cent, solution of 

 ammonio-ferric alum. The crystals of this salt should be clear 

 violet in colour; if they are yellowish and opaque, they have 

 suffered from exposure to air and are no longer fit for use. 

 The solution must be made cold, as the salt is decomposed 

 by heat. The slide is next washed with distilled water and 

 then placed in a i^ per cent, solution of Hcematoxylinwn purissi- 

 mum (Griibler). The over-stained sections are then again treated 

 with the iron-alum solution used before, in order to remove the 

 superfluous colour. The process of extraction must be followed 

 under the microscope and continued until thecell protoplasm is com- 

 pletely decolourised, and the chromatin network of the nucleus 

 becomes clear. One may interrupt the differentiating process any 

 moment by washing with fresh water, and then continue it. When 

 the extraction of the stain has been carried far enough, the slide 

 should be washed fifteen minutes in fresh water and mounted in 

 the usual way in balsam. 



Heidenhain noticed that when the differentiation was effected 

 quickly, the centrosomes were stained in greater number than when 

 the process occupied a long time. It seemed, therefore, that the 

 defects of the method might be corrected if a way could be found by 

 which the decolouring process could be hastened. How could the 

 cytoplasm be freed from the stain in the shortest time ? Assuming 

 that a stain acts by chemical combination, it seemed probable that 

 the process of extraction might be hastened, if the receptivity of 

 the cytoplasm could be at least partially saturated before the appli- 

 cation of the haematoxylin. Accordingly, Heidenhain selected as 

 preliminary stains (" Vorfarben ") such as afl'ect the cytoplasm and 



* Americcun Naturalist^ xxviii. (1894), PP- 97^ — 977- 



