and it contains many small granules in its cavity in addition to the nucleus. A transformation of the cell-contents then takes place, the granules gradually disappearing ; the wall of the cell at the same time becoming thicker at the expense of the cavity, so that eventually the latter completely disappears, and the entire secondary cell is converted into a solid cord. The cell-nuclei at first remain whilst this thickening of the cell-wall is going on, and become enclosed by it, rather than pushed into the cavity of the cell. They are at length entirely absorbed. Is, then, the thickening of the wall of the secondary muscle-cell a thickening of the cell-membrane itself, as appeared to be the case in cartilage? or is it a secondary deposit upon its inner surface, so that the cell-membrane is chemically and microscopically distinct from the substance, by means of which the secondary cell becomes converted into a solid cord? The latter is the more usual case in vegetables. The position of the cell-nuclei affords important evidence for the solution of the above question ; for as those bodies, gene- rally at least, lie firmly attached to the inner surface of the cell-membrane, they would be pushed towards the interior by a thickening of the cell-membrane itself, whilst a secondary deposit upon its inner surface, must enclose and fix them there, unless they should become separated altogether from the cell-wall. Now, in muscle, they actually remain lying in the circumference of the fasciculus, as represented by pl. IV, fig. 3, 6. This fact, then, renders it probable that the thickening of the wall of the secondary muscle-cells is due only to a secondary deposit. Such a supposition must, however, have been adopted, independent of the argument just raised, since the muscular fasciculi are, as it seems, enclosed by a structureless membrane. ‘The fasciculi have been long described as invested by a sheath, but that investment has been considered to be composed of cellular tissue, and to correspond in the primitive fascicul to the cellular tissue, by which the larger fasciculi are separated from one another. This membrane seems, however, to have quite a different signification, and to be the cell-membrane of the secondary muscle-cell. It is structureless, very transparent, and appears as a very narrow and sharply-defined border around each primitive fasciculus. I well know how readily such an appearance is produced by a
