74. Dr. G. C. Wallich on the Process of 
that the effect of plunging albumen (the substance most closely 
allied in character to sarcode) momentarily into strong spirit, 
acid, or even hot water, is to produce on its surface a hardened 
membranous-looking layer, which, as seen under the microscope, 
has all the appearance of the ectosare of Amba. It is a remark- 
able fact, moreover, that in those specimens of Ameba in which 
the ectosare presents the nearest approach in aspect to a mem- 
brane, under the application of a moderate degree of heat every 
trace of this vanishes, and the sarcode-mass becomes homogeneous 
to its extreme margin; whereas, in the encysted condition of 
Ameba, no heat, short of that capable of destroying the tissue 
altogether, suffices to alter the then strictly membranous character 
of the cyst*. 
One of the most characteristic features of sarcode is its ten- 
dency to vacuolation—that is to say, the formation of cavities 
within its substance, occupied by fluid or solid matter t. The 
first step in the process of spicular deposit is the formation of 
such a cavity, subject, of course, to variation in shape and size 
in different species, but the essential character of which remains 
the same in every instance. Taking for illustration, then, the 
simplest type of spicule above referred to, a correspondingly 
shaped vacuole makes its appearance in the sarcode-mass, but 
with this singular and constant peculiarity—that its long axis 
is traversed by a thread of sarcode, or vacuolar stolon, as I pro- 
pose to term it. This stolon is occasionally free at one of its 
extremities, but never at both. In the adjoining woodcut, a 
diagrammatic view is given of the order in which the successive 
layers of silex are deposited in a sponge-spicule, the upper end 
being closed, the lower open—the sectional view in each case 
being given immediately below. Fig. 1 represents the vacuolar 
cavity as seen in section longitudinally, s being the stolon, and 
v the space or cavity produced by endosmotic effusion of fluid 
containing silex in solution. There are present, therefore, two 
surfaces of sarcode, namely that of the cavity and that of the 
stolon. ach of these now secretes a layer of silex, not, how- 
ever, with an intervening space between, but in the closest ap- 
position. In fig. 2 these two layers are indicated by the horse- 
shoe-shaped spaces numbered 1 and 2 respectively, but which, 
in all probability, are formed simultaneously. 
Now, as each layer becomes immediately consolidated and is 
* These cysts may readily be mounted in balsam; but, in the ordinary 
condition of Amba it is impossible to preserve a vestige of outline when 
so mounted. 
+A vacuole may be defined as a space in a fluid of one density occupied 
by fluid or solid matter of another density. 
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