ROBERTS, ON BLOOD-CORPUSCLES, 173 
classes caused the macula to be invisible more frequently than 
in mammalia, inasmuch as it suffered eclipse when situated 
over or under the central nucleus. 
In the fowl, dace, and minnow, it was found easy to bring 
out the parietal macula; in the fish two spots were not un- 
frequently seen. The macula was situated indifferently on 
any part of the periphery, and sometimes it projected from 
the surface. When happily prepared, the specimens were 
even beautiful. The central nucleus was dyed of the finest 
red; and on the delicate outline of the cell-wall hung the red 
parictal macula, offering a not altogether fanciful resemblance 
to the astronomical figures representing the moon coursing 
in its orbit round the earth. 
At this stage of the inquiry it was conceived that an im- 
proved demonstration might be obtained by fixing the dye 
with a mordant, and then subjecting the corpuscles to a 
lavatory process, so as to get rid of the floating granules 
which so much interfered with the view. For this purpose 
a solution of tannin (which is one of the mordants for 
magenta used in the arts) was employed, and some advan- 
tage was found therein. When a solution of tannin, of 
three grains to the ounce of water, was added to blood that 
had already been dyed with magenta, it was found that the 
parietal maculz had their colour intensified, and that they 
became more conspicuous objects. The investigation was, 
however, not pushed any further im this direction, for it was 
found that tannin alone produced an even more remarkable 
effect than magenta. To this effect I now desire to draw 
particular attention. 
When a solution of tannin, of the strength of three grains 
to the ounce, was applhed to human blood, or to that of the 
horse, ox, sheep, pig, or cat, the blood immediately became 
turbid ;. and when a drop was placed under the microscope, 
the corpuscles were found greatly changed, as represented in 
fig. 2 
Each corpuscle appeared to have thrown out a bright, 
highly refractive bud or projection on its surface. The pro- 
jections were usually about a fourth part of the size of 
the corpuscle on which they were fixed; but they varied con- 
siderably. Some were only minute bright specks in the cell- 
wall; others were half or even two thirds as large as the 
corpuscle itself. Very rarely (in mammalian blood) two such 
This, doubtless, depended upon the varying densities of the liquor sanguinis 
and cell-contents in different animals,” 
