MICROSCOPICAL TECHNIC 427 



page 30 and shows that acid groups in the adsorbent fix color bases 

 especially; that basic groups fix acid colors; and that the amphoteric 

 amidonaphthol stains very well with both acid and with basic dyes. 



In the matter of cell staining not only is the chemical nature 

 of the cell constituents and of the dye involved, but the physical 

 prerequisites for the penetration of the dye must be supplied. 



In the dyes we have a group of substances, most of them chemically 

 well defined, which exhibit all transitions from true crystalloid (e.g., 

 methylene blue) to the highly colloidal hydrosols (e.g., benzoazurin). 

 There is, already, an extensive literature in reference to the colloidal 

 properties of dyes which has been collected in the work of L. PELET- 

 JOLIVET.* 



The experiments of R. HOBER and S. CHASSIN * showed that, in 

 general, the more colloidal a dye is, the more difficult is its absorp- 

 tion by the kidney epithelium of frogs. Some exceptions are prob- 

 ably associated with specific chemical properties. 



The idea that the staining of a living plant tissue depends on the 

 dispersion of the dye was developed into a comprehensive theory by 

 RUHLAND in his "ultrafilter theory" which points to a satisfactory 

 explanation of the staining processes in organized tissues. 1 



An important element in staining capacity seems to me to have 

 been disregarded hitherto; it is the question of the density of the sub- 

 stance to be stained in its relation to the diffusibility of the dye. It 

 is quite clear that an easily diffusible dye will penetrate everywhere, 

 and that a dye possessing no diffusibility will always remain on the 

 external surface of the tissue. If we are dealing with substances of 

 medium density, it will depend upon the density of the tissues 

 whether it will penetrate at all, and to what extent. If .we know 

 our dyes from this point of view we will be in a position to draw 

 conclusions from their penetration as to the structure of the tissue 

 examined. Some experiments performed with this end in view will 

 explain what is meant. 2 In solving the previous question I em- 

 ployed paper strips which were soaked in glacial acetic acid collodion 3 

 of different concentrations (8 per cent, 4.5 per cent, 1.5 per cent 



1 I am glad to learn that J. TRAUBE and F. KOHLER, obviously without know- 

 ing my views (see 1st edition of this book, 1912, p. 49), in a recent publication, 

 1915, likewise point out the significance of "the variability of the dispersing gel 

 by added substances," inasmuch as dyes producing swelling increase permeability 

 and dyes causing shrinking diminish it. I do not consider satisfactorily established 

 the proof of swelling and the shrinking properties on irreversible gels and the con- 

 clusion drawn from them since the experiments of the author depend only on 

 reversible gelatin gel. 



2 These investigations have not been published before. 



3 A solution of collodion in glacial acetic acid. 



