434 COLLECTED STUDIES IN IMMUNITY. 



VII. 



In the case of the chemically defined poisons, drugs, and dyes 

 discussed above, incorporation into the protoplasmic molecule does 

 not, barring a few exceptions, take place by means of synthesis. 

 Since, however, almost the greater part ot all substances foreign to the 

 body exhibit a typical selective action in the tissues, it becomes neces- 

 sary to study the reasons for this action. Here again we shall do best 

 to begin with a consideration of the phenomena which takes place 

 in staining reactions. A cotton fibre placed m a dilution of picric 

 acid of one to a million takes up the dye, becoming intensely stained. 

 Methylene blue introduced intra vitam into the organism is taken 

 up by the nerve endings. In poisoning by alkaloids certain nerve 

 centres may react specifically and alone. All of these phenomena 

 are obviously analogous in their nature. It seems necessary, therefore, 

 to discuss briefly the views held concerning the nature of the staining 

 process. The purely mechanical conception which refers it all to 

 physical processes, such as surface attraction and absorption, can 

 probably be discarded for the staining of substances in general. This 

 leaves only two other explanations, either of which may be the cor- 

 rect one for certain cases. 



The first of these, maintained particularly by Knecht, proceeds 

 from the assumption that certain constituents of the fibre substance 

 form with the dye insoluble salt-like combinations usually termed 

 laky combinations. This conception is supported by the fact that by 

 treatment with alkalies an acid can be obtained lanuginic acid 

 derived from wool, and nucleic acid from nuclear substances which 

 possesses the property of precipitating the salts of basic dyestuffs 

 even out of very dilute solutions. Analogous conditions are found 

 to a great extent in vital stainings. I need only remind the reader 

 of the investigations of Pfeiffer. These show that in the vital staining 

 of plant-cells one can frequently observe that the staining is due to 

 conspicuous granules of the almost insoluble tannate of methylene 

 blue. Naturally in the higher animals secretion substances present 

 in the cells and constituting precipitants which form laky combina- 

 tions can play a part in localization-. 



The second theory, one which associates the staining process 

 with the phenomenon of solid solutions, we owe to the researches 

 of O. N. Witt. This investigator starts with the fact that silk dyed 



