resemble one another in general character ; some are more irregular, more granulous, whilst others are relatively smooth. The smoother ones represent cylinders, which are generally more or less flattened (see pl. IV, fig. 3), in which they are delineated from the brachial muscles of a foetal pig seven inches in length, a representing the flat surface, b the marginal. The cylinder a presents a dark margin, and an internal clear portion, a distinction which is yet more manifest in c, where the dark margin is broader and sharply defined on its inner edge, so that it has quite the appearance of a hollow cylinder. I must, however, remark, that but very few fibres present this appearance sufficiently distinct to satisfy the mind of the ob- server. But in many instances it was so manifest, that no other explanation seemed left than to suppose the fibre a hollow tube. In the clear portion of the cylinder, which corresponds to the cavity, (in addition to some small granules,) larger oval corpuscles are seen, which are often very much extended in the longitudinal direction. Their form at once shows them to be nuclei, and they frequently contain one or two nucleoli. The distance at which they lie from one another is more or less regular in different instances. They do not lie in the axis of the fibre, but eccentrically, upon and within the thick- ness of the wall, as is seen when the fibre rests upon its margin. (See the fibre b.) That delineation exhibits a regu- larity in their position, since a nucleus lies upon the one side of the wall, the second on the opposite, and the third again upon the first side, and so on; such, however, does not appear to be the case in every instance. The nuclei are flat, for when viewed edgeways they have the appearance of mere stripes. The thickness of the wall of the cylinder seems to vary, as is shown by a comparison of a with c. The latter, c, the wall of which is the thicker, already presents an appearance of transverse striae. The nuclei, however, are also still visible in it, as well as small isolated globules which are contained in its cavity. Muscular fibre does not present any appearance of a cavity after the period of development before mentioned has passed, but the nuclei remain visible for a long time, lying in the thickness of the fibre, and often project upon the outside in the form of small prominences.
The other form of muscular fibre is delineated in pl. IV,
