92 COCHINEAL AND OTHER ORGANIC STAINS. 



thin sections and delicate objects are best stained in a very 

 dilute solution. 



A nuclear stain, slightly affecting protoplasm. The colour 

 varies with the reaction of the tissues, and the presence or 

 absence of certain salts. The salts of the metals and alkaline 

 earths that are present in the tissues, and that are soluble in 

 alcohol, give rise to colourations of a bluish tone, so that when 

 these are present the effect is that of a haematoxylin stain. 

 In the presence of acids of course the precipitation of these 

 blue combinations in the nuclei and protoplasm cannot occur, 

 and therefore tissues of an acid reaction, as well as those free 

 from the salts in question, stain red. Crustacea with thick 

 chitinous integuments are generally stained red, most other 

 organisms blue. The stain is also often of different colours 

 in different tissue elements of the same preparation. Glands 

 or their secretion often stain grey-green. In embryos of 

 Lumbricus Kleinenberg found the vessels to stain red, their 

 contents of an intense blue. 



Acids lighten the stain and make it yellowish-red. Caustic 

 alkalies turn it to a deep purple. 



The best stains are obtained in the case of objects that have 

 been prepared with chromic or picric acid combinations, or 

 with absolute alcohol.* The acids must be carefully washed 

 out before staining, or a diffuse stain will result. The stain is 

 permanent in oil of cloves and balsam. 



The object for which this stain was imagined is twofold. 

 Firstly, to obtain an alcoholic stain which enables us to do 

 away with the necessity of treating with an aqueous fluid 

 objects that have been preserved in alcohol and that are 

 intended for mounting in balsam, aqueous fluids being often 

 most deleterious to delicate structures. Secondly, to obtain a 

 fluid whose high penetrating power allows it to be employed 

 in the case of organisms, such as Arthropoda, whose chitinous 

 investments are but very slightly permeable by aqueous solu- 

 tions of carmine. 



I have treated this stain at considerable length, because I 

 am convinced that it ought to be better known. It is very 

 useful in many cases (Annelids, for instance), and indis- 

 pensable for Arthropoda. 



* Osmic acid preparations stain very weakly unless they have been pre- 

 viously bleached (No. 542). 



