THE TWO GREAT GROUPS OF CONNECTIVE-TISSUE CELLS. 9 



When now we enter the realm embraced by studies on the physical peculiarities 

 of dyes in relation to their power to affect the living cell, we are confronted at once 

 by the predominant theory in this field, that of Overton, which would recognize 

 solubility in lipoids as a necessary possession of any coloring substance which can 

 gain entrance into the cell. If adequate as an explanation for the behavior of most 

 of the vital stains originally discovered (methylene blue, neutral red), this theory 

 must be abandoned in the case of staining with the great acid-azo group — dyes 

 insoluble in lipoids, but soluble in water. There may, indeed, still be some per- 

 tinency in Overton's suggestion, inasmuch as the most rapid penetration by the 

 basic dyes of the protoplasm of all cells may be referable to the solubility of 

 these dyes in certain substances; certain it is that the acid-azo compounds gain 

 admittance only to a precise set of cells and are apparently forbidden entrance into 

 the great group of nervous, muscular, and epithelial tissues. Nor is the rate of 

 entry of the acid dyes into the cells which they peculiarly affect (the macrophages) 

 anything like as speedy a process as that shown by basic dyestuffs. In a few 

 seconds after treatment with weak yellowish solutions of neutral red these cells 

 concentrate in their segregation-apparatus deep red deposits of this dye, but they 

 are unable to effect such a concentration of the acid dyes, even when swimming in 

 high concentrations of the latter, except after an interval of several hours. 1 



It is apparent that, if the acid dyes enter the macrophages by free diffusion, 

 the latter process is at least seriously impeded when compared with the diffusion of 

 basic dyes. We have long been acquainted with a slower, though effective, manner 

 of entry of substances into the living cell- — phagocytosis — and it could seem no mere 

 accident that acid-dye granules were exhibited predominantly by cells whose phago- 

 cytic properties have long been known (Metchnikoff, Renaut, Dubreuil). We have, 

 indeed, through experiments with particles visible with the oil lens, and which, of 

 course, settle quickly out of suspension (lampblack), with particles barely on the 

 limits of visibility and which settle more slowly (India ink), with suspensions which 

 are permanent and the particles of which are invisible except to the ultra-microscope 

 (the metallic sols), and with high colloidal dyes of the acid-azo series, been in 

 the possession of a series of substances whose physical dimensions start from those 

 comparable with bacteria and which possess no power of independent movement, 

 and, indeed, merely oscillate with Brownian motion, to particles whose existence 

 can be displayed only by their opacity and the associated Tyndall phenomenon 

 of their "solutions" or by the ultra-microscope, to end with particles which can 

 penetrate membranes by dialysis and reach considerable distances through their 

 power of free diffusion. All of these substances enter and are concentrated in the 

 segregation-apparatus of the macrophage cells, where the conditions for their 

 anchorage are sufficient to allow them to remain for a considerable time. While 



1 Winternitz and Evans have shown that as a consequence of mechanical injury (tapping of the cover glass) blood-colls 

 are instantly, though diffusely, and palely stained by the acid dyes. It is probable that this change in behavior is due to 

 the "setting" phenomena which occur on protoplasmic death and which immediately allow the free spread of acid dyes, 

 though no concentration of the same. The latter is a phenomenon exhibited only by the living cell and is the only out- 

 standing justification of the term "vital staining" when applied to dyes considered here, for their concentration in the 

 protoplasm can only be carried out by the living cell. Loele (1912) has shown a similar great delay in the rate at which 

 paramecia house pyroll blue in their vacuoles when compared with methylene blue. 



