72            THE MICROTOMIST' S VADE-MECUM
earths that are present in the tissues, and that are soluble in 4 stal
alcohol, give rise to colourations of a bluish tone, so that when
these are present the effect is that of a hematoxylin stain      b
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   i
from the salts in question, stain red. Crustacea with thick        N1textr
chitinous integuments are generally, stained red, most other       ske40l, '
organisms blue. The stain is also often of different colours        u ti
in different tissue elements of the same preparation. Glands       ti sr
or their secretion often stain grey green. In embryos of            d
Lumbricus Kleinenberg found the vessels to stain red, their        kprdera
contents of an intense blue.
Acids lighten the stain and make it yellowish red. Caustic       Petrati
alkalies turn it to a deep purple.                                  a83
The best stains are obtained in the case of objects that have     eelis h
been prepared with chromic or pieric acid combinations, or         thdal
with absolute alcohol.'   The acids must be carefully washed        sreiti
out before staining, or a diffuse stain will result. If it is        at
wished to have the protoplasm strongly stained as well as the       aUdh
nuclei, it is only necessary to wash out incompletely after         jaRM
staining, and to fix the colouring matter by means of strong        haige'
alcohol. The stain is permanent in oil of cloves and balsam. agetible
The object for which this stain was imagined is twofold.          oltho
Firstly, to obtain an alcoholic stain which enables us to do        which
away with the necessity of treating with an aqueous fluid eabiti
objects that have been preserved in alcohol and that are             This
intended for mounting in balsam, aqueous fluids being often          stEat
most deleterious to delicate structures. Secondly, to obtain a       o ndi
fluid whose high penetrating power allows it to be employed          etst
in the case of organisms, such as Arthropoda, whose chitinous        tnes2
investments are but very slightly permeable by aqueous solu-         t
tions of carmine.                                                     Y
I Osmic acid preparations stain very weakly unless they have been
previously bleached (No. 476).


K