THE MODE OF ACTION OF VITAL DYES 295 



Other vital dyes can in certain cases be fixed in separate cell- 

 inclusions. It is claimed that mitochondria can be coloured vitally 

 by methylene blue and that the dye is retained if pieces of tissue 

 are fixed in a solution of formaldehyde containing ammonium 

 molybdate and picric acid.^^^'^^^ Certain granules in mammalian 

 blastomeres colour metachromatically with toluidine blue during 

 life.^^^ These granules are tentatively identified as large mito- 

 chondria. The metachromatic colouring can be fixed by a solution 

 of mercuric chloride containing ammonium molybdate and 

 phosphotungstic acid; it is retained in balsam. -^^ Neutral red is 

 said to be fixed by Altmann's fluid. ^^^ 



(For the use of trapping agents in non-vital work, see p. 224.) 



