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 ectosarc of Amoeba. It is a remark- 

 able fact, moreover, that in those specimens of Amoeba in which 

 the ectosarc 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 

 Amoeba, 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 f. 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. Each 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 Ammba it is impossible to preserve a vestige of outline when 

 so mounted. 



t A vacuole may be defined as a space in a fluid of one density occupied 

 by fluid or solid matter of another density. 



