CHAPTER XL 129 



207. Intra-vitam Staining. It is clear that unless the cell-membrane 

 of a living cell is permeable to a dye, no constituent of the cell can 

 be stained by it. Most dyes appear to be more or less toxic if they 

 enter the cell. But, while alive, the latter is to a large extent 

 protected, since the dye does not obtain entrance. A living Amceba 

 is stained by very few dyes. Neutral red, however, passes through 

 the membrane and stains various structures, while having no 

 apparent effect on the activities of the organism. The auricle of 

 the frog's heart can also be stained with this dye, while continuing 

 its normal contractions. Used in this way, the dye is applied in very 

 dilute solution. Since neutral red is a very sensitive indicator just 

 about the neutral point, the fact of its permeability and non-toxicity 

 makes it a valuable test for the presence of acid or alkali within the 

 cell. 



When a dye enters a living cell, it usually stains various granules 

 and structures contained therein, while at the same time it is uni- 

 formly diffused through the liquid phase of the protoplasm. If the 

 process of staining is conditioned by phenomena at boundary sur- 

 faces, simple undifferentiated protoplasm in the living state should be 

 incapable of staining, and this seems to be the general experience. 

 As regards the question of permeability to a given dye, unless the 

 cell is able to show that it is still alive by movement or by con- 

 tractility, it is clearly a matter of difficulty to be certain that, when 

 a particular dye enters, it does so during life or only after it has 

 destroyed the normal properties of the cell-membrane. The nucleus 

 itself seems to be very resistant to dyes while alive, and it has been 1 

 stated that the appearance of stain in it is a sure indication of death. ) 

 BOLLES LEE made a large number of observations and came to the 

 conclusion that most of the " intra-vitam " stains are either due to 

 mere diffusion through the liquid protoplasm or that the stained 

 constituents were not really living, being food particles or products 

 of cell activity. 



At the same time, many of the methods which come under this 

 heading are of much value. Methylen blue may be injected into 

 the living animal and frequently gives very successful staining of 

 nervous structures, owing to the fact of its being conveyed into 

 intimate contact with the cells by means. of the blood vessels. 



The various methods of preserving the stain in the structures to 

 which it was localised during life obviously depend on the adequacy 

 of the means used to fix and maintain these structures and to retain 

 the properties owing to which the stain was taken up. This is by 

 no means a simple matter. MOTT describes in living nerve cells a 



