fig. 1, from the dorsal muscles of a foetal pig of three inches and a half in length. They are in general somewhat thicker than those last described, more irregular, not so smooth, but more granulated. The existence of a special wall to the fibre and of a cavity in its interior, may be quite as distinctly, or even more clearly, recognised in many of these. (See the fibre c in fig. 1.) The wall is not so smooth as in the other form of muscular fibre. The contents are always very granulous. Distinct cell-nuclei, and not unfrequently nucleoli also, even in the natural state, may often be perceived in them. Com- monly, however, only the circular or oval outlines of the nuclei are distinctly perceptible, in consequence of the other granules which are contained in the cavity of the fibre lying above them, and the general granulous nature of the fibre renders an accurate discernment of the nucleus particularly difficult. But if a drop of acetic acid be added, the fibre becomes perfectly transparent, and swells; the nuclei, on the contrary, remain dark, shrivel up slightly, and may then be distinguished with perfect accuracy. This is exemplified by fig. 2, which represents the fibre c of fig. 1 after having been treated with acetic acid. The indubitable cell-nuclei, partially furnished with nucleoli, are there seen, with isolated small dark granules between them. The nuclei have indeed under- gone a slight change from the acetic acid, but they do not all present a regular aspect even in the recent state. The majority of them are flat. In the recent state, some appear to be placed on their edges, presenting an appearance as though the cavity of the fibre were divided into compartments by small thick transverse striae. The nuclei he much nearer together in this than in the form of muscular fibre previously described, so that the distance of the central points of two nuclei from one another, is generally equal to, or even less than, the thickness of the fibre.
This second form of muscular fibre appears to be an earlier condition of the first. The younger the embryo the more abundant is this form of fibre, and it gradually becomes less so as development proceeds. ‘The steps of this transition may readily be conceived. The fibre becomes extended in its entire length, is thereby rendered thinner, the cell-nuclei are removed farther from one another, and in some instances
