XXX INTEODUCTION. 



fixation and washing. In other cases (e.g. the fresh-water 

 mussel) a weak solution of chromic acid (|- per cent.) in water 

 serves well. For the brain of the rabbit and pigeon ordinary 

 methylated spirit is convenient. 



3. Staining not only makes the tissues more distinct, but 

 also affords insight into their minute structure. 



Most stains will not penetrate a living organism or tissue. 

 Methylene blue, however, is an exception, and therefore of 

 great value. It is employed in very dilute solutions (see 

 Appendix) for the study of small transparent forms {e.g. Para- 

 mecium, Hydra) and for the tracing out of nerves and nerve- 

 cells in higher animals. For rapidly staining dead but fresh 

 tissues and such as have been fixed and washed in water, 

 safranin and dahlia are useful, especially for Protozoa, 

 developing spermatozoa of the earthworm, leech, and crayfish. 

 The stain must only be allowed to act for two or three minutes, 

 and then the surplus carefully removed by distilled water. 



Carmine is a generally useful stain, especially Grenacher's 

 borax-carmine. As this is an alcoholic stain, specimens 

 should be placed in it from 50 per cent, alcohol. After the ^ 

 staining is completed the object should be transferred to acid- 

 alcohol for a few minutes or hours according to its size. 



Picrocarmine is an aqueous solution, and directly appli- 

 cable to objects washed after fixing. Treat subsequently with 

 50 per cent., 70 per cent., and acid-alcohol. 



Hfematoxylin (especially Meyer's acid-hasm-alum, see 

 Appendix) gives excellent results if used with due care, but a 

 trace of acid or turpentine may completely destroy the colour. 



X. ON CUTTING SECTIONS. 



Many points in anatomy and histology may be best made 

 out by examination of microscopical sections, that is, slices 

 cut so thin as to allow of their being examined under the micro- 

 scope by transmitted light. 



The hardened object, supported, if necessary, between two 

 pieces of carrot, may be held between the fingers and thumb 

 and sections sliced off it freehand with a sharp razor. 



