136 



Picric acid. If desired, and it will often be advisable, to 

 follow the nuclear stain (any one of the preceding) with a 

 secondary stain, picric acid can be employed to best advan- 

 tage. It is of special value in insect histology because it 

 clearly distinguishes chitin. A little picric acid should be 

 added to a small jar of 96$ alcohol and the slide passed 

 through it before going into the pure 96$ alcohol. This 

 bath of picric alcohol must come after the bath of acidulated 

 alcohol used in differentiating after Mayer's acidulated car- 

 mine. 



Heidenhains iron hcematoxylin (Lee's Vade-Mecum, p. 189). 

 This is a more complicated method of haematoxylin staining, 

 which, however, gives excellent and beautiful results and is 

 very helpful in those cases where cellular structure is being 

 studied. The staining is done as follows : Put sections for 

 from one to two hours in a 2% to 4$ aqueous solution of fer- 

 ric alum (NH 4 ) s Fe a (SO 4 ) v This mordants them. Then 

 wash the sections in water for from five to ten minutes. 

 Then stain for from one-half to two hours in a y z % aqueous 

 solution of haematoxylin (made by taking 3 c.c. of a 16$ 

 alcoholic solution of haematoxylin, chloral hydrate 2 grams, 

 and distilled water 97 c.c.). Then wash the sections in 

 water, and differentiate by dipping the slide for a few sec- 

 onds in the mordant (the ferric alum solution) and then 

 rinsing in tap water, repeating this operation until the cor- 

 rect differentiation has been attained as ascertained by ex- 

 amination of the wet slide under the microscope. The 

 chromatin should be deep blue or blue black, the cytoplasm 

 gray or light blue. After differentiating, the slide should be 

 washed in running water for fifteen or twenty minutes to 

 remove completely the ferric alum. Considerable practice is 

 necessary to obtain the best results with this stain. It 

 should not be used with thick sections, i.e., thicker than 10 

 microns, for it is an opaque stain. 



