" PICRO-CARMINATE OF AMMONIA." 253 



tion of the picro-carmine contained in the glycerine is not 

 of much consequence, provided there is an excess of picric 

 acid present. The colours of specimens mounted in this way- 

 improve very much for some time after they are put up. 



Specimens hardened in chromic acid or Miiller^s fluid, 

 likewise those which have been kept long in osmic acid, do 

 not stain so easily as others. The picro-carminate of am- 

 monia stains the tissues in several colours, varying from a 

 bright red to an intense yellow. It colours yellow the horny- 

 layer of the epidermis, the central cells in the bird's nests of 

 epithelioma, hairs, nails, cartilage (very slightly), and elastic 

 fibres ; nuclei of cells are coloured bright red ; the fibres of 

 connective tissue of a rose colour; the protoplasm of most 

 cells takes a reddish-yellow colour ; and, lastly, the red blood- 

 corpuscles assume a brown tint.^ 



In the epidermis, for example, the picro-carminate dis- 

 tinguishes three layers : — 



I. Most superficially the horny layer, consisting of 

 flattened cells, coloured of an intense yellow. 



II. An intermediate layer of cells filled with granules, 

 which are coloured of a bright red. 



III. And, lastly, a layer of cells under this coloured of a 

 dull yellow, with their nuclei stained red. 



These three layers can often be distinguished in the bird's 

 nests of squamous epithelioma, the centre containing the 

 intensely yellow cells, outside this is the red layer, and, most 

 externally, the dull yellow cells with red nuclei. This 

 indicates plainly the development of the epidermal cells 

 inwards, passing through the three stages. 



The great advantage the picro-carminate has over other 

 staining fluids is its property of staining tissues in a series of 

 colours varying from red to yellow ; it has the minor ad- 

 vantages of colouring rapidly and equally, and of being able 

 to be kept in the dried form. 



I may mention that in Prey^s ' Microscopic Technology ' 

 (4th German edition) picro-carminate of ammonia is alluded 

 to in a few lines, but only the original method of preparation 

 employed by M. Ranvier is given. 



' These colours will vary slightly, according to the quality of the Picro- 

 Carmmate. 



