138 MUSCLE. 
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 ap- 
peared to be the case in cartilage? or is it a secondary deposit 
upon its inner surface, so that the cell-membrane is chemi- 
cally 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 imner 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 mner 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 struc- 
tureless 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 fasciculi 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 
