MICROSCOPICAL TECHNIQUE. 109 



the nucleus, and leave the centrosomes unstained. Thus, the che- 

 mical affinities of the centrosomes for the haematoxylin would 

 remain at full strength ; while those of the cytoplasm and nucleus 

 would be more or less saturated, and to the same extent weakened 

 for the hsematoxylin. In this way the process of extraction was 

 brought under some control, and the method greatly improved. 



Stains reached in this way are called " subtractive." Bordeaux 

 R., Anilin blue and Methyl-eosin were employed as preliminary 

 stains. Bordeaux R. proved to be the best. In preparations that 

 have been successfully differentiated as to the centrosomes, the 

 nucleus and its chromatin are almost colourless, so that the cen- 

 trosome may be easily studied, even when it lies behind the 

 nucleus. The nucleoli remain strongly stained. 



The Chromatin. — Heidenhain shows that there are two kinds 

 of chromatin to be distinguished — namely, an oxychromatin brought 

 out by acid anilin stains {e.g,, Rubin S.), and a basichromatiii, which 

 is brought out by basic anilin stains {e.g., Methyl green). The "basi- 

 chromatin " is the chromatin of Flemming and authors in general. 



The differentiation of the two chromatins can only be accom- 

 plished when the nucleus is exposed at the same time to both acid 

 and basic anilin colours, as is the case when Biondi's solution and 

 Ehrlich's triacid are used. 



If one mixes ammonium vanadate with haematoxylinum pur 

 (Griibler), a blue stain is obtained which stains cytoplasm and oxy- 

 chromatift strongly, while the basichromatin is often left nearly 

 colourless. The two chromatins probably differ only in the 

 amount of phosphorus present, basichromatin containing more, 

 oxychromatin less. 



Aluminium has the property, when used as a pencil, of leaving 

 an indelible mark on glass or any other substance having a siliceous 

 base. A deposition of the metal takes place, and, while this may 

 be removed by a suitable acid wash, the mark itself cannot be 

 removed by rubbing or washing. Magnesium, zinc, and cadmium 

 have a similar property, but the mark of magnesium is easily 

 removed, the application of zinc requires a wheel, and zinc and cad- 

 mium tarnish ; while aluminium is permanent and remains bright. 



