dyes safranin and gentian violet, thus staining the chromatin with 

 gentian violet and the rest of the nucleus with safranin. It is 

 difficult to say just how any chemical theory of staining can yet 

 satisfactorily explain such selective action as this. It is, indeed, 

 admitted by some upholders of the chemical theory that the chem- 

 ical action of dyes is not specific, and merely serves to differentiate 

 acid from basic elements of the tissue; and that the further differ- 

 entiation, as between chromatin and other parts of the nucleus, is 

 due to physical forces, thus indicating a difference in the structure 

 rather than in the chemistry of the different portions of the nucleus. 

 The neutral stains have a very interesting action. When tissue 

 is stained with them they seem to break up partlj^ into their com- 

 ponent acid and basic elements and stain portions of the tissue as 

 the simple dyes would individually. But in addition the neutral 

 stain itself seems to have an affinity for certain parts of the tissue 

 and hence a third color is possible. This explains in part the poly- 

 chrome effects obtained by the eosin-methylene-blue blood stains — 

 only partly so, however, because in preparing these stains methy- 

 lene blue breaks up into certain other dyes so that more shades than 

 those expected from eosin and methylene blue alone are obtained. 

 So far as the action of stains is chemical, their use forms a con- 

 necting link between the two subjects, histology and micro- 

 chemistry. These two branches of science are generally thought 

 to be entirely distinct. The histologist, with the technic and view- 

 point of the biologist, prepares delicate sections of various materials 

 colored with one or more of a long series of available dyes, and 

 studies the biological structures present under the microscope. The 

 microchemist, with the technic and vie^\'point of the chemist, 

 examines with the microscope similar material treated with various 

 reagents of known chemical reaction, and from his observations 

 draws conclusions as to the chemical nature of the substances 

 examined. There is some possibility, however, that the difference 

 between histology and microchemistry is one of point of view rather 

 than of methods. Both the microchemist and the histologist 

 study the action of chemical compounds on substances or struc- 

 tures visible under the microscope; the difference is that the micro- 

 chemist uses the chemical compounds in question as chemical re- 

 agents while the histologist uses his as dyes to color the microscopic 

 structures and thus to increase their visibilitv. 



Now, on the chemical theory of staining, the biologist is using 

 complicated chemical reactions in his microscopic technic — so 

 complicated in fact as to be unintelligible chemically. To bridge 

 the gap between histology and microchemistry these reactions 

 must be made intelligible. The first step in this direction has now 

 been taken by Unna (1921) in a very important contribution to 

 the subject of cell chemistry. He points out the need of harmoniz- 

 ing chemical and histological investigations and proposes a method 

 of doing this which he calls chromolysis. The technic he has de- 



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