cells of the tissues of the following classes, in most instances, do not advance beyond this early stage in the development of the feather-cells, but the changes necessary to the formation of the subsequent tissues occur at this period; their cellular nature is, however, quite as certain as is that of the young feather-cells, although it be not possible to recognize their cell-wall so clearly as in their perfectly-developed condition.
The matter contained in the cells is either a transparent fluid, as in the cells of the pith of feathers previously to their becoming dry, or in the crystalline lens, when it contains albumen ; or, a minutely-granulous mass, as in many epithelium-cells, or pigment-granules; or, it is altogether absent, and the cell-walls, in consequence of their flattening, are in immediate contact. The air in the cells of the pith of mature feathers simply penetrates from without, during the process of their desiccation. With the exception of some of the cells of the lens, all the cells of this class are invariably furnished with a nucleus of the characteristic form. It is not, however, a persistent structure, as in the previous class, but in very many instances becomes absorbed when the cells have reached maturity; such is the case in the pith of the feather, the superior laminae of the epidermis, the nails, crystalline lens, &c. &c.
As a general rule the cells remain independent during all these changes, that is to say, each cell retains its especial wall, and its own peculiar closed cavity. More or less complete blendings of the cell-walls, and even of their cavities also, occur, however, as exceptions even in this class. The epithelial scales of the nail are so intimately connected together, that it is rarely possible to trace the contour of one of them in its entire circumference; and the same appears to be the case with the epithelium in the vessels of the adult. The coalescence, however, does not appear to be perfect, for, by the employment of concentrated acids, the scales of the nail may be separated somewhat more readily from each other. A union of the cavities of several cells seems to occur in the pigment-cells. A prolongation of a cell filled with pigment may be seen to pass uninterruptedly to the cavity of another cell (plate II, fig. 9, a). In such an instance, probably, the prolongations of two cell-cavities join at a certain point, the cell-walls unite together there, and the partition-wall becomes absorbed, and
