720 MEDICAL ENTOMOLOGY 



the Bless' fluid is replaced by seventy per cent alcohol, and subsequently 

 by eighty per cent, ninety per cent, and absolute alcohol, allowing ten 

 minutes in each of the two first, and one hour in the last ; it is then 

 taken back to forty-five per cent alcohol, allowing a few minutes in each 

 strength. The dilute alcohol is then replaced by haematoxylin stain, 

 which is allowed to act for several hours. All dissections should be 

 overstained, and decolorized with dilute acid one-quarter per cent acetic 

 or hydrochloric under the microscope. The length of staining and 

 deco.orization depend, of course, on the stain and the density of the 

 tissue. Several hours are almost always necessary. When decolorization 

 is complete the acid is replaced by forty-five per cent alcohol, and the 

 preparation carried through the same series as before to absolute alcohol, 

 cleared in clove oil, and mounted in Canada balsam. 



It is often convenient to allow a certain number of dissections to accu- 

 mulate before staining them, as a dozen or so can be carried through 

 with little more trouble than a single one. They keep perfectly well in 

 Bless' fluid. In decolorizing, select the least dense of the batch and 

 allow it to remain on the stage of the microscope in its solid watch 

 glass examining it from time to time. The remainder need not be 

 looked at until this is ready. As each specimen is decolorized it should 

 be carried up to seventy per cent alcohol, and allowed to remain there till 

 all are ready, when they can be mounted together. Decolorization may 

 take many hours ; it should not be hastened unduly by using a stronger 

 acid, or the sharpness of the stain will be lost. 



Instead of haematoxylin, borax carmine stain may be used. It gives 

 excellent results. 



Brilliant results can be obtained -by this method by noting the avidity 

 with which different tissues take up the stain, and so adjusting the 

 period of application and the decolorization as to bring out the special 

 point desired. For instance, muscle fibre takes up carmine readily, but 

 is easily decolorized, so that if a piece of gut is stained for only a few- 

 hours, and then carefully decolorized, the circular and longitudinal 

 fibres are beautifully shown up, the cells remaining unstained. By treat- 

 ing another piece for a longer time, and preferably with haematoxylin, the 

 cells in the wall can be shown without any of the muscles. Similarly 

 with the ovaries, a few experiments will enable one to show up the nurse 

 cells, the apical filament, or the marginal epithelium. All three cannot 

 be shown satisfactorily on the same preparation. Every tissue has its 

 own peculiarities, which must be learned by experience, but successful 

 preparations show so much more than can be learned from fresh tissues, 



