In the Flounder, which may serve as a type, the primitive symmetry is very soon destroyed by the appearance of fin rays on the ventral side. The region where they are present soon forms a lobe; and an externally heterocercal tail is produced (fig. 36 A). The ventral lobe with its rays continues to grow more prominent and causes the tail fin to become bilobed (fig. 36 B); there being a dorsal embryonic lobe without fin rays (_c_), which contains the notochord, and a ventral lobe with fin rays, which will form the permanent caudal fin. In this condition the tail fin resembles the usual Elasmobranch form or still more that of some Ganoids, _e.g._ the Sturgeon. The ventral lobe continues to develop; and soon projects beyond the dorsal, which gradually atrophies together with the notochord contained in it, and finally disappears, leaving hardly a trace on the dorsal side of the tail (fig. 36 C, _c_). In the meantime the fin rays of the ventral lobe gradually become parallel to the axis of the body; and this lobe, together with a few accessory dorsal and ventral fin rays supported by neural and hæmal processes, forms the permanent tail fin, which though internally unsymmetrical, assumes an externally symmetrical form. The upturned end of the notochord which was originally continued into the primitive dorsal lobe becomes ensheathed in a bone without a division into separate vertebræ. This bone forms the urostyle (_u_). The hæmal processes belonging to it are represented by two cartilaginous masses, which subsequently ossify, forming the hypural bones, and supporting the primary fin rays of the tail (fig. 36 C). The ultimate changes of the notochord and urostyle vary very considerably in the different types of Teleostei. Teleostei may fairly be described as passing through an Elasmobranch stage or a stage like that of most pre-jurassic Ganoids or the Sturgeon as far as concerns their caudal fin. The anterior paired fins arise before the posterior; and there do not appear to be any such indications as in Elasmobranchii of the paired fins arising as parts of a continuous lateral fin. Most osseous fishes pass through more or less considerable post-embryonic changes, the most remarkable of which are those undergone by the Pleuronectidæ[22]. These fishes, which in the adult state have the eyes unsymmetrically placed on one side of the head, leave the egg like normal Teleostei. In the majority of cases as they become older the eye on the side, which in the adult is without an eye, travels a little forward and then gradually rotates over the dorsal side of the head, till finally it comes to lie on the same side as the other eye. During this process the rotating eye always remains at the surface and continues functional; and on the two eyes coming to the same side of the head the side of the body without an organ of vision loses its pigment cells, and becomes colourless. [22] _Vide_ Agassiz (No. 56) and Steenstrup, Malm.