64 Transactions of the Eoyal Microscopical Society. 
adjustment at the proper focus, it will be observed to represent an 
even border, about yo^o in breadth, which is distinguished 
from the rest of the body by a decidedly greenish tint (Fig. 1). 
With an oblique illumination, the existence of this greenish border 
becomes still more evident, so that even an inner contour, in the 
form of a delicate shady line, may be recognized. It is true, this 
inner contour is not comparable to the inner contour of a distinct 
cell-wall ; nevertheless, it is sufficiently distinct to indicate the 
presence of a thin layer at the surface of the blood-corpuscle, 
differing in some respects from the rest of the body — as, for 
instance, in the refraction of light. In the accompanying drawings, 
this greenish border of the corpuscle is, for the sake of distinction, 
represented clear, while the yellow portion of the body is shaded. 
It will be obvious that this observation is an important one ; 
indicating, as it does, the existence of a thin layer at the surface 
of the blood-corpuscles, differing, if not in chemical composition, 
at least in density from the substance of the disks. 
This layer may have, as Lankester remarks, no pronounced 
inner limitation, and may not, in the true sense of the word, 
possess the character of an enveloping membrane ; but under 
certain conditions its connection with the underlying material 
is lost, and it appears in the form of a true membrane, manifesting 
itself by a double contour. Such conditions are especially brought 
about by the influence of certain reagents, as we shall see here- 
after. But the same phenomenon is now and then also observed 
in the corpuscles of blood just escaped from the vessels. Thus, it 
sometimes happens that we meet in perfectly fresh blood, which 
has been quickly removed from the vessels and transferred to 
a slightly warmed slide, with specimens of blood-corpuscles, in 
which the protoplasm of the body of the corpuscles has, by a spon- 
taneous contraction, become separated to a limited extent from the 
overlying denser layer representing the surface ; so that the latter 
manifests itself by a fine double contour. In these instances, with 
the exception of the small vacuum produced by the retraction of 
the protoplasm, the blood-corpuscle has lost nothing of its normal 
appearance ; its main body retains its slightly dirty, greenish- 
yellow colour, while the superficial layer or stratum ap])ears in 
the form of a slight greenish border, the continuity of which over 
the vacuum is manifested by a fine double contour. 
The phenomenon just described, I have not only observed in the 
blood-corpuscles of the Amphiuma, but also in those of the Frog 
(Fig. 49). All further doubts regarding the existence of a distinct 
stratum, or membraneous layer on the surface of these blood- 
corpuscles, differing in density if not in composition from the rest 
of the body, ought to vanish in the face of such self-evident facts. 
A very singular observation which I made while examining a 
