The lateral line itself presents some points of not inconsiderable interest. In the first place, it is very narrow anteriorly and throughout the greater part of its length, but widens out at its hinder end, and is widest of all at its termination, which is perfectly abrupt. The following measurements of it were taken from an embryo belonging to stage L, which though not quite my second youngest embryo is only slightly older. At its hinder end it was 0.17 Mm. broad. At a point not far from this it was 0.09 Mm. broad, and anteriorly it was 0.05 Mm. broad. These measurements clearly shew that the lateral line is broadest at what may be called its growing-point, a fact which explains its extraordinary breadth in the anterior part of the body at my first stage, viz. 0.28 Mm., a breadth which strangely contrasts with the breadth, viz. 0.05 Mm., which it has in the same part of the body at the present stage. It still continues to form a linear area of modified epidermis, and has no segmental characters. Anteriorly it is formed by the cells of the mucous layer becoming more columnar (Pl. 13 fig. 3_a_). In its middle region the cells of the mucous layer in it are still simply elongated, but, as has been said above, have a gable-like arrangement, so as partially to enclose the nerve (Pl. 13, fig. 3_b_). Nearer the hind end of the trunk a space appears in it between its columnar cells and the flattened cells of the outermost layer of the skin (Pl. 13, fig. 3_c_), and this space becomes posteriorly invested by a very definite layer of cells. The space (Pl. 13, fig. 3_d_) or lumen has a slit-like section, and is not formed by the closing in of an originally open groove, but by the formation of a cavity in the midst of the cells of the lateral line. Its walls are formed by a layer of columnar cells on the inner side, and flattened cells on the outer side, both layers however appearing to be derived from the mucous layer of the epidermis. The outer layer of cells attains its greatest thickness dorsally. During stages M, N, O, the lateral nerve gradually passes inwards into the connective tissue between the dorso-lateral and the ventro-lateral muscles, and becomes even before the close of stage N completely isolated from the lateral line. The growth of the lateral line itself remains for some time almost stationary; anteriorly the cells retain the gable-like arrangement which characterised them at an earlier period, but cease to enclose the nerve; posteriorly the line retains its original more complicated constitution as a closed canal. In stage O the cells of the anterior part of the line, as well as those of the posterior, commence to assume a tubular arrangement, and the lateral line takes the form of a canal. The tubular form is due to a hollowing out of the lateral line itself and a rearrangement of its cells. As the lateral line becomes converted into a canal it recedes from the surface.