130 CHAPTKK XI. 



or on one or other of the elements that go to make up the 

 cytoplasm. 



Stains that thus exhibit a selective affinity for the sub- 

 stance of nuclei nuclear or chromatiu stains form a class 

 of stains of peculiar importance for the embryologist or 

 zootomist. For they enable him to have the nuclei of 

 tissues marked out by staining in the midst of the unstained 

 material in such a way that they may form landmarks 

 to catch the eye, which is then able to follow out with ease 

 the contours and relations of the elements to which the 

 nuclei belong. 



To these must be added another group of stains of the 

 greatest importance to the cytologist and histologist, the 

 plasmatic stains or plasma stains. These take effect es- 

 pecially on elements of cells and tissues other than the 

 chromatin for instance, on the reticulum of cytoplasm, 

 or on its granules, or on polar corpuscles, etc., or on the 

 formed material of tissues. 



203. Dyes : Basic, Acid, and Neutral. The colouring matters, 

 or dyes, employed either in industrial dyeing or in histo- 

 logical staining are almost always salts. They are known 

 as " basic," " acid," or "neutral ;> dyes. By a " basic " dye 

 is meant one which is either a base or a compound in which 

 a so-called " colour base ; ' (or molecular group to which the 

 compound owes its colouring properties) is combined with a 

 non-colouring acid. For instance, fuchsin or magenta is a 

 basic dye. It is the hydrochloride of rosanilin, and its 

 colouring properties are due to the rosanilin which exists as 

 a base in the compound, and not to the hydrochloric acid of 

 the compound. By an " acid ;i dye is meant a compound in 

 \vhich a so-called " colour-acid ' is combined with a non- 

 colouring base. The dye known as acid fuchsin or acid 

 nmgeiita (Siiurefuchsin) is an " acid ' colour. It is the 

 soda-salt of di- or tri-sulphoconjugated rosanilin, that is of 

 rosanilin di- or tri-sulphonic acid, and its colouring pro- 

 perties are due to the rosanilin which exists as an acid in 

 the compound, and not to the soda. Or, to take a simpler 

 case, picrate of ammonia is. an " acid ' colour, and its 

 colouring properties are evidently due to the picric acid in 

 it, and not to the ammonia. 



