"The animal cell, in its most complete form, is comparable in most parts of its structure to that of the plant, but differs from it in the entire absence of the 'cellulose wall' or of anything that represents it, the cell-contents being enclosed in only a single limitary membrane, the chemical composition of which, being albuminous, indicates its correspondence with the primordial utricle. In its young state it seems always to contain a semi-fluid plasma, which is essentially the same as the protoplasm of the plant, save that it does not include chlorophyll granules, and this may either continue to occupy its cavity (which is the case in cells whose entire energy is directed to growth and multiplication) or may give place, either wholly or in part, to the special product which it may be the function of the cell to prepare. Like the vegetable cell, that of animals very commonly multiplies by duplicative sub-division, it also (especially among Protozoa) may give origin to new cells by the breaking up of its contents into several particles." Some of these creatures are so exceedingly minute that hundreds of millions may be contained in a drop of water. They have no organs of any kind, but consist of single cells of a soft substance called "sorcode;" there is no distinction of sexes, and generation takes place by sub-division, each cell separating into two, and these again into two more, and so on with marvellous rapidity; this is the usual method of increase, but occasionally an act of conjugation takes place, two cells blending into one at the points of contact, the result of which is a nucleus or cell within the original one. These animated cells are of all forms, having prolongations which appear to be thrown out to absorb any particle of organic matter within reach, which then enters within the soft substance of the animalcule and is digested or dissolved, such are the Amboeba, the Actinophrys, &c. (see fig. 1); a step or two higher from these, the very lowest, infusoria are found to possess the first rudiments of organs in the form of cilia, which consist of minute elongations having a constant vibratory motion, for the purpose of causing a current in the water in which they live and bringing food into their vicinity.