202 CHAPTER XVI. 



body^ its " leucobase." Now living or recently dead tissue 

 elements are, or may be, botli alkaline and very greedy of 

 oxygen, and thus act on tbe dye as reducing agents. The 

 leucobase thus formed is easily reoxidised into methylen blue 

 by oxidising substances, or acids, or even by the mere 

 contact of air — which latter property is taken advantage of 

 in practice. 



It follows that a total stain of all the tissues of a living 

 intact organism can hardly be obtained under these con- 

 ditions, 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. If it be 

 one that stains later than the others, it may be studied 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 § 34.3. 



. The proper strength of the very dilute solutions to be 

 employed for the staining of living organisms 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 in- 

 creased or diminished as occasion may seem to require. 

 ZojA {Rendic R. 1st. Lombardo, xxv, 1892 ; Zeit. wiss. Mile, 

 ix, 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. 

 The stain penetrates very badly, which is no doubt one cause 

 of its capriciousness. Gland cells generally stain early ; 

 then, in no definable order, other epitheliuui 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 performed 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, ivhich 



