Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/110

FEATHERS. are most completely developed resemble the cells which we have seen in the mature feather (see plate II, fig. 11, a) in every other respect, but that they lie less firmly connected together, so that they may readily be isolated, and the peculiar wall of each cell be distinctly seen. The walls are of sufficient thickness to prevent their losing their angular shape, even when in the isolated state. There are intercellular spaces between some of them, and such also occur between the fully-developed cells of the perfect feather. The cell-membrane is dark and smooth, and the cell-contents consist of a transparent fluid. A very distinct nucleus is also seen lyimg upon the wall of each cell. It is oval, and contains one or two nucleoli, which are large in proportion to its size (see the figure). There is no nucleus to be seen in the fully-developed cells, and it is only in very rare instances that its remains can be detected; it must therefore undergo absorption at some subsequent period. ‘The process may, indeed, be followed; for example, the cells which form the stages of transition between those delineated in fig. 11 a, and fig. 10, are more closely connected together, the nucleus m them becomes smaller, and its outline more irregular, the nucleolus meanwhile re- mains; at length both disappear. The degree of development attaimed by the cells is generally proportionate to their dis- tance from the matrix ; and as that is situated on the inner side of the feather, at the part where the shaft exhibits a furrow, those cells which he immediately under the cortex, at the back of the shaft, are the most perfectly developed. Now the cells when traced from that part inwards towards the ma- trix, are found to become gradually smaller; the cell-mem- branes lose their dark outlines, and present a granulous aspect. The nucleus of the larger granulous cells has still the same form as in those previously described with a smooth cell-membrane; its size, however, diminishes with that of the cell. The cells in this granulous condition resemble most of the elementary cells of other tissues. Plate II, fig. 11, b, c, repre- sents the stages of transition. Advancing still nearer towards the matrix, the cells are no longer recognizable; all that we can see being numerous nuclei, which he close together in a minutely-granulous substance (plate II, fig. 12).

The process of formation of the cells of the pith is, there-