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This class presents us with the greatest similarity between animal and vegetable structure, and, indeed, in so high a degree, that even an experienced botanist cannot distinguish some of the objects which belong to it from vegetable tissue. Most animal cells may be distinguished from the mature vegetable cells by their greater softness and delicacy; but those characteristics are in some measure wanting in this class, and it would be very difficult to distinguish microscopically between a thin layer cut off from the interior of the shaft of a feather and a portion of vegetable tissue. We shall, therefore, take the feather as our example, and endeavour to trace these cells, which correspond in so striking a manner with vegetable tissue, backwards to their primitive condition, explaining this transition by delineations, and in this way convince ourselves that, in their early stage, they also accord with the primitive cells of all other tissues. The tissues comprised under the term horny belong to this class, and the crystalline lens may also be included in it. The cells of these tissues generally remain independent, but more or less intimate blendings of the cell-walls with one another also occur in this class. Horny tissue may be reduced to two unessential subdivisions, viz.—1. Its membranous expansions, to which belong the Epithelium, in the extended sense of the term (including the Epidermis), and the Pigmentum nigrum, which must be enumerated here, in consequence of its intimate alliance with the epithelium. 2. The compact horny formations, including the Nails, Claws, Hair, Feathers, &c.

1. Epithelium.—It is very difficult to determine what this term ought to comprise. The cortical substance of the chorda dorsalis, which is composed of flattened hexagonal cells (in the larva of Rana esculenta, for example), cannot be regarded as epithelium, since it is made up of the same cells as those of the interior of the chorda dorsalis: the sole difference consisting in