METHYLEN BLUE. 79 



colourless. If now you put back the animals into the dilute 

 solution and wait, you will find on examination after a suffi- 

 cient lapse of time that further groups of tissues have become 

 stained. Thus it was found by EHELICH (Abh. Jc. Akad. Wiss. 

 Berlin, February 25, 1885) to whom the principle of the 

 methods under consideration is due, that on injection of the 

 colour into living animals axis cylinders of sensory nerves 

 stain, whilst motor nerves remain colourless. The motor 

 nerves, however, will also stain, though later than the sensory 

 nerves. It might be supposed that by continuing the staining 

 for a sufficient time, a point would be arrived at at which all 

 the tissues would be found to be stained. This, however, is 

 generally not the case. It is always found that the stained 

 tissues only keep the colour that they have taken up for a 

 short time after they have attained the maximum degree of 

 coloration of which they are 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 those 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 coloration 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. 



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 observed 

 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 not to be an obstacle to the observation of the 

 later stained ones; the latter being either at the point of 

 maximum coloration or at a point of desired intensity either 



