THE FROG 101 



tion of Congo red). Stain deeply with the hsematoxylin, if 

 necessary leaving the slides in the stain over night, then 

 destain in acid alcohol (0.25 per cent, hydrochloric acid in 

 70 per cent, alcohol) until the required differentiation is ob- 

 tained. In case orange G or Congo red are used as counter- 

 stains, the slides must be left in the stains over night and 

 then passed directly to -the higher alcohols, otherwise the 

 stain will wash out. Staining on the slide is a time-consum- 

 ing process in case a large number of slides are to be pre- 

 pared, but the practical certainty of getting good results, 

 and the permanency of the stains, make this method de- 

 cidedly to be preferred for most tissues of the frog. (&) 

 A more rapid method, so far as the actual work is concerned, 

 is to stain the material in to to in Grenacher's borax-carmine 

 for a day or two, and destain for several hours, to remove 

 the stain from the cytoplasm, in acid alcohol. Be careful 

 not to destain too long; the process should be stopped as 

 soon as the object changes from a dull red to a slightly 

 pinkish hue. Counterstain on the slide with Lyons blue to 

 which sufficient picric acid has been added to change the 

 color to green. Be careful not to counterstain too deeply, 

 and wash in at least one change of xylol. This counterstain 

 gives remarkably good differentiation of the cytoplasmic 

 structures, but it will fade within a few years. In order to 

 stain the nuclei with sufficient intensity in the borax-carmine, 

 be careful to get a good preparation of this stain, and if 

 necessary stain for two days, keeping the material in a warm 

 place. The object should have an acid reaction. Certain 

 objects (e. g., the stomach and kidney) after fixation in 

 Zenker's sometimes take this stain very poorly. 



The paraffin method of imbedding and cutting is to be 



