230 CHAPTER XVII. 



susceptible ; as soon as that point is attained they begin to discharge the 

 colour even more quickly than they took it up. And it is very often found 

 that the elements which have stained first will have lost much or all of their 

 colour by the time that tliose which stain later have attained their maximum 

 coloration. It may even happen, as I have observed, that the whole of the 

 stainable tissues of an animal may run through the total gamut of colora- 

 tion and decoloration until the animal has become as colourless as when 

 first put into the tinted water, and that without any apparent change in its 

 vital activities. The stain, therefore, that has thus been produced and lost 

 is not a true histological stain, but a quasi-stain (see 201). 



It follows that a total stain of all the tissues of an organism can hardly 

 be obtained under these conditions, but that a specific stain of one group or 

 another of elements may be obtained in one of two ways. If the tissue to 

 be studied be one that stains earlier than the others, it may be studied 

 during life at the period at which it alone has attained the desired intensity 

 of coloration, and the remaining tissues are not yet coloured at all, or not 

 coloured enough to be an obstacle to observation. If it be one that stains 

 later than the others, it may be studied during life at the period at which 

 the earlier stained elements have already passed their point of maximum 

 coloration and have become sufficiently decoloured ; the later stained ones 

 being at a point of desired intensity. Or the observer may fix the stain in 

 either of these stages and preserve it for leisurely study by means of one of 

 the processes given below under the heading " Preservation of the Prepa- 

 rations." 



The proper strength of the very dilute solutions to be employed in the 

 manner here considered must be made out by experiment for each object. I 

 think the tint is practically a sufficient guide, but it may be stated that when 

 in doubt a strength of 1 : 100,000 may be taken, and increased or diminished 

 as occasion may seem to require. ZOJA (Rendic. R. 1st. Lombardo, xxv, 

 1892 ; Zeit.f. wiss. MiJc., ix, 2, 1892, p. 208) finds that for Hydra the right 

 strength is from 1 : 20,000 to 1 : 10,000. 



The stain is capricious. It is not possible to predict without trial which 

 tissues will stain first in any organism. It is to be remarked that the stain 

 penetrates very badly, which is perhaps the chief cause of its capriciousness, 

 and, I take it, the chief determining condition of the order in which tissues 

 stain. Gland cells generally stain early; then, in no definable order, other 

 epithelium cells, fat cells, plasma cells, " Mastzellen," blood and lymph 

 corpuscles, elastic fibres, smooth muscle, striated muscle. There are other 

 elements that stain in the living state, but not when the staining is per- 

 formed by simple immersion of intact animals in a dilute staining solution 

 in the manner we are considering. Chief amongst these are nerve-fibres 

 and ganglion cells, which remain unstained in the intact organism, most 

 likely, so far as I can see, for the simple reason that the stain is not able to 

 penetrate them. 



324. Staining Nervous Tissue during Life. As stated above, 

 it was made out by EHRLICH that by injecting a solution of 

 methyl en blue into the vessels or tissues of living animals 



