588 PROTOZOA 



Carmine does not usually stain paraffin sections easily 

 (Laxgerox, Precis de Microscopic, 1913, p. 369), and material 

 such as human liver, containing Ilistoplasma capsulatum, or 

 salivary duct of mule infected with Cryptococcus farcinimosus 

 (fixed in formalin) sometimes proves impossible to stain with 

 carmine. For sections of these I have obtained better results 

 by staining with safranin (a saturated solution in distilled water 

 for ten minutes) instead of carmine. It tends to wash out during 

 the differentiation with clove oil, but after the excess of blue 

 colour has been removed differentiation can be stopped by clearing 

 in xylol and the nuclei left a brick-red colour, as well as any 

 connective tissue fibres and yolk present, while the parasites 

 (Gram positive) alone are blue. 



TESTS FOR CHROMATIN 



1159. Methyl green in presence of an acid is useful for staining 

 chromatin in fresh unfixed tissues (see Chapter XVII., p. 178). 



For sections fixed in sublimate, the same stain contained in 

 the Ehrlich-Biondi-Heidenhain mixture may be used for chromatin 

 (§ 322). 



This method is purely a staining process and is not a very 

 definite one either. It is often very difficult with granules, 

 especially small cytoplasmic ones, to determine whether or not 

 they acquire a greenish tint. 



1160. Feulgen's Reaction — a Microchemical Test for Chromatin 

 See Chapter XXVII. and Woodcock {Journ. Roy. Army Med. 

 Corps, May, 1926, p. 1) ; Robertson {Parasit., xix, 1927, p. 375). 

 Strictly speaking this is a test for thymonucleic acid, a constituent 

 of chromatin of animal cells. It consists in hydrolysing certain 

 purin-bodies contained in this acid, and breaking them down 

 into groups of aldehyde nature. The test therefore consists of 

 two parts : (1) hydrolysis, (2) the application of Schiff's reaction 

 (fuchsin-sulphurous acid) for the presence of aldehydes. See § 628. 



By this reaction any chromatin (containing thymonucleic 

 acid) should have taken on a pink to violet tint, which is remark- 

 ably permanent and resistant, and the cytoplasm should be 

 colourless. 



Yeast cells do not give the reaction, because their chromatin 

 contains a pcntosenucleic acid, not thymonucleic acid. The same 

 appears to be true of many plant nuclei. 



Woodcock has also had negative results with Sarcocystis and 

 the spores of Glugea lophii, but the microsporidian Thelohania sp. 

 gave the reaction, and we have obtained it with Thelohania mulleri 

 spores from Gammarus. 



Resting nuclei of Gregarines sometimes give no reaction. 



An interesting observation made by Robertson is that the 



