The fact of the epiblast of the neural canal being divided, like the remainder of the layer, into nervous and epidermic parts, cannot, I think, be used as an argument in favour of the opposite view to that here maintained. It seems probable that the central canal of the nervous system arose as an involution from the exterior, and therefore that the epidermis lining it is in reality merely a part of the external epidermis, and as such is naturally separated from the true nervous structures adjacent to it[196]. Footnote 196: Vide Self, "Development of Spinal Nerves in Elasmobranchii." _Phil. Transact._ 1876. [This Edition, No. VIII.] Leaving the general features of the external skin, I pass to the special organs derived from it during the stage just anterior to K. _The unpaired Fins._ The unpaired fins have grown considerably, and the epiblast composing them becomes, like the remainder of the layer, divided into two strata, both however composed of more or less columnar cells. The ventral fin has now become more prominent than the dorsal fin; but the latter extends forward as a fold quite to the anterior part of the body. _The paired Fins._ Along each side of the body there appears during this stage a thickened line of epiblast, which from the first exhibits two special developments: one of these just in front of the anus, and a second and better marked one opposite the front end of the segmental duct. These two special thickenings are the rudiments of the paired fins, which thus arise as special developments of a continuous ridge on each side, precisely like the ridges of epiblast which form the rudiments of the unpaired fins. Similar thickenings to those in Elasmobranchii are found at the ends of the limbs in the embryos of both Birds and Mammals, in the form of caps of columnar epiblast[197]. Footnote 197: For Birds, vide _Elements of Embryology_, Foster and Balfour, pp. 144, 145, and for Mammals, Kölliker, _Entwicklungsgeschichte_, p. 283.