MUSEUM OF COMPARATIVE ZOOLOGY. 175 



tions cannot be studied to advantage until they have been depigmented. 

 For this purpose I know of only two classes of successful reagents, acids 

 and strong alkalis. Grenacher has generally employed the first, Graber 

 the second. 



Of the acid reagents strong solutions are required. Lankester and 

 Bourne ('83, p. 180) employed 5 or 10^ solutions of nitric acid. In 

 the eyes which I have studied, this mixture did not remove the pigment, 

 even after the lapse of a week ; and I was forced to use stronger and 

 stronger grades, till 50% was reached. This mixture gives fair results, 

 but must be made and used with much caution. A given volume of acid 

 should be poured slowly into an equal measure of alcohol, never the re- 

 verse, and the mixture should be kept cool, otherwise the acid may attack 

 the alcohol. In such an event the solution is rendered worthless, and, 

 should the specimens be in it at the time, the heat generated by the 

 reaction gives the acid such additional dissolving power that the sections 

 are at once destroyed. A more efficient acid reagent is a mixture of 

 equal parts hydrochloric and nitric acids. A 35 % solution of this mix- 

 ture in strong alcohol gives better results than the pure nitric acid at 

 50%, and does laot so readily attack the alcohol. 



Of the alkalis, weak ammonia, sodic hydrate, and potassic hydrate 

 are most serviceable. The solids are to be preferred to the ammonia, 

 since from them solutions of a definite strength can more easily be made. 

 An aqueous solution of J or ^% potassic hydrate has given the most 

 satisfactory results. 



The method of using the depigmenting fluid is as follows. Unstained 

 material is cut in paraffine ; the ribbons are mounted on a slide with 

 Schallibaum's fixative ; when the sections are fixed, the paraffine is re- 

 moved with turpentine ; the slide with the sections is then successively 

 washed with alcohol of 98%, 90%, 70%, and so on, till a grade homo- 

 geneous with the depigmenting fluid is reached. Into a shallow white 

 dish filled with the depigmenting fluid the slide is now gently lowered. 

 In z^few seconds the pigment, dissolving, will be seen as a reddish cloud. 

 The process is usually completed in less than a minute, and the slide is 

 promptly transferred to a dish of clean water or alcohol and there gently 

 rinsed. The sections are next stained by exposure to the dye in a shal- 

 low dish. After being sufficiently stained, they may be washed and 

 mounted in glycerine, or, after the proper steps in dehydrating and clari- 

 fying, mounted in benzol-balsam or other mounting medium. 



The dyes which have been found the most serviceable are some of the 

 carmines and haematoxylin. The aniline dyes have almost invariably 



