101 
Histology and Embryology. By C. S. Minot. 
If now tlie sections, after being thus freed from the adherent 
foreign matter, be mounted directly, they make poor preparations ; 
the single parts are indistinct, and the whole is very transparent. 
This can be avoided by colouring them. It may be safely asserted 
that the introduction of staining fluids, by Grerlach, in 1858, was 
the most important step in advance ever yet made in histological 
technic. Colouring matters, as regards their action on cells, belong 
to two classes : either they produce a diffuse colouring of the whole 
cell, or they stain the nucleus much more deeply than the proto- 
plasm and the membrane of the cell. The principal are dyes of 
the latter class, carmine, hsematoxiline, and aniline blue, which are 
esteemed in the order named. The two former are invaluable, for 
by marking out the nuclei so distinctly they enable us to recognize 
so many centres of cells, and to observe characters which have been 
made prominent by their colouration, and are very different in the 
various forms of cells. In fact, preparations for the microscope 
cannot be felt or dissected, but only seen ; therefore the differential 
colouring produced by carmine, for example, is an assistance to the 
eye, comparable to the raised alphabets of the blind. In both 
cases, the conditions under which the special sense, whether sight 
or feeling, has to act are greatly exaggerated, so to speak, thus 
producing magnified or strengthened perceptions. 
Carmine is by far the most generally useful. It is employed in 
various solutions, the recipes for which may be found in various 
handbooks, and need not, therefore, be quoted in this article. The 
first step in preparing it is to dissolve some of the fine-powdered 
carmine in a small quantity of ammonia, and it may be used at once 
in that form after allowing the superfluous ammonia almost entirely 
to evaporate. A very excellent solution may be prepared by simply 
adding an equal volume of rather strong acetic acid to the dissolved 
carmine ; the exact proportion is not of very great import. Beale’s 
carmine keeps a long time without alteration, and Kanvier’s picro- 
carmine has certain advantages ; but on the whole, I have found 
the above-mentioned mixture of acetic and ammoniacal carmine to 
be quite sufficient for most work. 
Hsematoxiline, on the other hand, has to be employed in a 
particular solution. Dissolve first thirty-five parts of hsematoxiline 
crystals in one thousand parts of absolute alcohol, and mix it cold 
with a solution of ten parts alum in three thousand parts distilled 
water. The mixture is purple at first, but turns a deep blue in the 
course of a few weeks ; but it may be used without waiting for the 
change of colour. For use it must always be filtered through 
porous paper to free it from sediment, and it may be advantageously 
diluted with 0 • 5 per cent, solution of alum. It acts much more 
quickly and produces a deeper and more exclusive staining of the 
nuclei than does carmine. It is therefore particularly applicable in 
those cases where it is desired to study the shape and transforma- 
