FEATHERS.
fore, as follows: a minutely-granulous mass is present in the first instance, in which numerous cell-nuclei lie, some of them exhibiting a nucleolus. Around them the cells are formed, being at first not much larger than the nuclei, and having a granulous aspect. The cells gradually expand; the nucleus also grows, and soon reaches its full maturity. It remains eccentrical, lying upon the cell-wall. The cell-membrane retains its granulous aspect for a time; gradually losing it, however, as the expansion of the cells advances; at the same time the contents of the cell-membrane become darker, but the cell-walls are not at all diminished in thickness. The walls of the cells, in the next place, become more firmly united together, so that they cannot be separated from one another so readily, and at the same time the nucleus gradually dis- appears. The contents of the cells at last dry up, and they become filled with air. The development of these cells accords, therefore, entirely with the vegetable cells, the nucleus being their true cytoblast; it is present before the cell, and, as is generally the case in the cells of plants, afterwards becomes absorbed. The cell expands, growing by intussusception, and the membrane of the fully-developed cell might, without much danger of error, be assumed to be more than ten times heavier than that of the youngest one. The physical, and probably also the chemical, condition of the cell-membrane undergoes a change. The cytoblastema, in which the cell-nuclei are in the first place formed, consists of granules, analogous to the mucus-granules, in which, according to Schleiden (Müller’s Archiv, 1838, plate III, fig. 2), the cytoblasts of vegetable cells originate. According to Schleiden, those mucus-granules are deposited from a solution of gum within a parent-cell. The cells of feathers are not formed in parent-cells, but in the neighbourhood of the organized matrix. There can be no doubt, however, but that the matrix only exudes a fluid, which afterwards becomes transformed into a granulous substance. I have not investigated the mode in which the nuclei originate in the cytoblastema, whether by a junction of smaller globules, whether the nucleoli first exist, and so forth. The growth of the nucleus proceeds for a time with that of the cell; for the latter is formed around the nucleus before it has reached its full size. The cytoblas-
