In the anterior parts of the body during stage L the skin only presents two distinct strata, viz. an inner somewhat irregular layer of rounded cells, the mucous layer, and an outer layer of flattened cells (Pl. 13, fig. 8). The remaining history of the external epiblast, consisting as it does of a record of the gradual increase in thickness of the epidermic strata, and a topographical description of its variations in structure and thickness in different parts, is of no special interest and need not detain us here. In the late embryonic periods subsequent to stage Q the layers of the skin cease to be so distinct as at an earlier period, partly owing to the innermost layer becoming less columnar, and partly to the presence of a large number of mucous cells, which have by that stage made their appearance. I have followed with some care the development of the placoid scales, but my observations so completely accord with those of Dr O. Hertwig[228], that it is not necessary to record them. The so-called enamel layer is a simple product of the thickening and calcification of the basement membrane, and since this membrane is derived from the mucous layer of the epidermis, the enamel is clearly to be viewed as an epidermic product. There is no indication of a gradual conversion of the bases of the columnar cells forming the mucous layer of the epidermis into enamel prisms, as is frequently stated to occur in the formation of the enamel of the teeth in higher Vertebrates. Footnote 228: _Jenaische Zeitschrift_, Vol. VIII. _Lateral line._ The lateral line and the nervous structures appended to it have been recently studied from an embryological point of view by Götte[229] in Amphibians and by Semper[230] in Elasmobranchii. Footnote 229: _Entwicklungsgeschichte d. Unke._ Footnote 230: _Urogenitalsystem d. Selachier._ Semper's _Arbeiten_, Bd. II. The most important morphological result which these two distinguished investigators believe themselves to have arrived at is the direct derivation of the lateral nerve from the ectoderm. On this point there is a complete accord between them, and Semper especially explains that it is extremely easy to establish the fact. As will appear from the sequel, I have not been so fortunate as Semper in elucidating the origin of the lateral nerve, and my observations bear an interpretation not in the least in accordance with the views of my predecessors, though not perhaps quite conclusive against them. It must be premised that two distinct structures have to be dealt with, viz. the _lateral line_ formed of modified epidermis, and the _lateral nerve_ whose origin is in question. The lateral line is the first of the two to make its appearance, at a stage slightly subsequent to K, in the form of a linear thickening of the inner row of cells of the external epiblast, on each side, at the level of the notochord.