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AMOEBA
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AMOEBA , the See also:Greek See also:equivalent of the name " Amibe " given by fiery St See also:Vincent to the See also:Proteus animalcule of earlier naturalists, used as a quasi-popular See also:term for any See also:simple naked protist the See also:sole See also:external See also:organs of which are pseudopodia, i.e. temporary outgrowths of the clearer See also:outer layer of the soft protoplasmic See also:body. It is also used as a generic name, and in its See also:present limitations by E. Penard includes only those the pseudopodia of which are constantly changing, See also:blunt outgrowths. In the former wider sense, amoebae are found in sluggish See also:waters, fresh and See also:salt, all over the See also:world; they readily make their See also:appearance in infusions putrefying after infection from aerially carried germs, and the leucocytes or colourless See also:blood corpuscles of Metazoa are essentially amoebae in their structure and behaviour. The See also:protoplasm of the individual is divided into a centrally placed body, the See also:nucleus, of relatively See also:stable shape, and the cytoplasm, itself divided into an outer, clearer ectoplasm (" ectosarc ") and an inner, more granular endoplasm (" endosarc "), passing into one another. The movements of amoebae are of several kinds. (1) The amoeba may grow out irregularly into blunt lobes, the pseudopodia, some being emitted while others are retracted, and so may advance in any direction by the emission of pseudopodia thitherward, and the enlargement of these by the passage of the organism into them. (2) Again, it may advance by a sort of See also:rolling: the See also:lower See also:surface, or that in contact with the sub-stratum over or under which it is passing, is viscid and adheres to the substratum, the superficial dorsal layer passing forward and bending over to the ventral See also:side; whilst the converse See also:action takes See also:place at the hinder end; (3) or again, the pseudopodia, when See also:long, well marked and relatively permanent, may serve as actual limbs on which the body is supported and on which it moves. In the outgrowth of a See also:pseudopod the See also:process may take place gradually, the ectoplasm growing as it stretches, or it may take place by the limiting layer of the ectosarc bursting, as it were, and a rounded prominence of the endosarc protruding and at once forming a new " skin " or pellicle. This last mode, termed " eruptive," is See also:common in the See also:case of the enormous, multinucleate amoeba termed See also:Pelomyxa palustris, which attains a See also:diameter when contracted and spherical of as much as a See also:line (over mm.). From the ease with which amoebae are obtained and kept alive under the See also:microscope, as well as from their identity in structure with the See also:primitive elements of Metazoa, they have always been favourite See also:objects of study for protoplasmic See also:physiology under its simplest conditions. Among the investigators of protoplasmic movements we may cite F. Dujardin, O. Biitschli, L. Rhumbler and H. S. Jennings. The opening to the exterior of the contractile vesicle has been found here. Pelomyxa has yielded to A. E. See also:Dixon and M. Hartog a peptic ferment, such as has been extracted by C. F. W. Krukenbergfromthe Myxomycete Fuligo (See also:Flowers of Tan), which is the largest known naked'See also:mass of protoplasm without cellular differentiation. Amoeba shows also the multiplication by fission, so characteristic of the See also:cell: for the study of other modes of See also:reproduction, spore formation and syngamic (or so-called fertilization) processes, fresh-See also:water or salt-water amoebae are See also:ill suited, and up to this date we do not know the See also:life See also:cycle of any See also:free-living naked amoeba, though that of some parasitic forms and See also:shell-bearers have been fully made out. Some amoebae are certainly See also:young states of Myxomycetes. Encystment, the See also:excretion of a membrane around the cell to See also:tide over unfavourable circumstances, has been noted in almost all See also:species. Amoeba coli and A. histolytica are parasites in the gut of See also:man, the former relatively harmless, the latter the cause of severe See also:dysentery and hepatic See also:abscess, common in See also:India. H. S. Jennings has recently made a full study of the movements of Amoeba, and of its See also:general behaviour, and found therein many indications that these are on the whole such as we should expect of an organism working by " trial and See also:error " rather than the See also:uniform modes of non-living beings. Thus the operations of an amoeba ingesting a See also:round, encysted Euglena are summed up thus: " One seems to see that the amoeba is trying to obtain this cyst for See also:food, that it shows remarkable pertinacity in continuing its attempts to put forth efforts to accomplish this in various ways, and that it shows remarkable pertinacity in continuing its attempts to ingest the food when it meets with difficulties. Indeed the See also:scene could' be described in a much more vivid and interesting way by the use of terms still more anthropomorphic in tendency." (M. Additional information and CommentsThere are no comments yet for this article.
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