INTRODUCTION TO VITAL COLOURING 283 



When a living cell, stained supervitally, has been under examina- 

 tion for an hour or so, without any precautions having been taken 

 to keep it alive, its appearance begins to change. Water often 

 separates from close association with the proteins of the cytoplasm 

 and appears in the form of vacuoles. These vacuoles have a ten- 

 dency to colour w^th neutral red. It is important to distinguish 

 betw^een these new formations and the pre-existing bodies that 

 have a particular affinity for this dye. At length, whether this 

 vacuolation has taken place or not, certain irreversible changes 

 occur, which are made very evident by the presence of the dye. 

 Previously, as has been remarked, the cell bore little resemblance 

 to what is seen in a permanent preparation. Now, suddenly, that is 

 no longer so. A network appears in the nucleus and takes up the 

 dye strongly, and cytoplasmic inclusions that w^ere coloured in 

 life become invisible. 



In general, one should not expect a dye to give the same picture 

 when used vitally as it gives in fixed preparations. This is not 

 simply because fixation and embedding dissolve out certain colour- 

 able cell-inclusions and change the reactions of others. In fixed 

 preparations the dye penetrates most of the tissues at the concen- 

 tration at which we present it, and w^e determine the end-point of 

 its reaction, either by removing it w^hen a desired effect has been 

 produced, or by diflFerentiating. In vital colouring, on the contrary, 

 as de Beauchamps ^^ pointed out half a century ago, our solvent is 

 not the solvent in which the dye is presented to the objects con- 

 tained in cells, nor can we directly control the concentration at 

 which the dye will act. A state of equilibrium is gradually built up 

 between the dye as we present it and the fluid of the cell, and that 

 equilibrium is not under our control. 



