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Rh mere optical deception, and that one can never be positive with respect to it unless it be observed that the margin in question does not accurately follow every bend of the fasciculus. It is, therefore, difficult to be convinced of this in mammalia; but in all those larvæ of insects which present the broad transverse striae of the fasciculi, discovered by Müller, the membrane, when the continuity of the proper muscular substance of a primitive fasciculus has been broken at a certain point, may be distinctly observed passing over uninterruptedly from the one portion to the other. Pl. IV, fig. 4, represents such a fasciculus; the membrane encompasses it so loosely (this larva had been preserved in spirits of wine) that a portion of the muscular substance could even change its position within the cavity. The membrane, where entirely isolated from the other parts of the preparation, shows itself to be quite structureless, and, indeed, the sharply-defined external contour renders it very improbable that it should be composed of areolar tissue. I, therefore, consider it extremely probable that it represents the cell-membrane of the secondary muscle-cell. It thus not only serves to isolate the fasciculi, but forms an essential constituent part of them. Pl. IV, fig. 5, exhibits this structureless membrane upon a muscular fasciculus of the pike; this preparation, however, was not quite convincing, inasmuch as the inferior edge of the fasciculus was covered by muscles lying above it. By means of this membrane, the muscular fasciculus remains, throughout its entire existence, a cell with a closed membrane and a cell cavity, the latter being filled with a firm substance, the peculiar muscular substance. It, therefore, clearly follows from the above that nervous fibres cannot pass between the primitive fibres (fibrils) of muscle; and that the latter cannot separate from their fasciculi, so as to pursue a more extended and independent course, as is common with fibres of areolar tissue; since, in either case, the cell-membrane must be ruptured.

The true muscular substance, which is thus, in the first place, formed as a secondary deposition upon the inner surface of the secondary muscle-cell, and continues to be so deposited until the entire cavity is filled, is composed in its mature condition, of very minute longitudinal fibres, the so-called primi-