The separation of the solid nervous system from the epiblast takes place relatively very late; and, before it has been completed, the first traces of the auditory pits, of the optic vesicles, and of the olfactory pits are visible. The auditory pit arises as a solid thickening of the nervous layer of the epiblast at its point of junction with the medullary keel; and the optic vesicles spring as solid outgrowths from part of the keel itself. The olfactory pits are barely indicated as thickenings of the nervous layer of the epiblast. [FIG. 32. TWO TRANSVERSE SECTIONS OF SYNGNATHUS. (After Calberla.) A. Younger stage before the definite establishment of the notochord. B. Older stage. The epidermic layer of the epiblast is represented in black. _ep._ epidermic layer of epiblast; _mc._ neural cord; _hy._ hypoblast; _me._ mesoblast; _ch._ notochord.] At this early stage all the organs of special sense are attached to a layer continuous with or forming part of the central nervous system; and this fact has led Götte (No. 63) to speak of a special-sense plate, belonging to the central nervous system and not to the skin, from which all the organs of special sense are developed; and to conclude that a serial homology exists between these organs in their development. A comparison between Teleostei and other forms shews that this view cannot be upheld; even in Teleostei the auditory and olfactory rudiments arise rather from the epiblast at the sides of the brain than from the brain itself, while the optic vesicles spring from the first directly from the medullary keel, and are therefore connected with the central nervous system rather than with the external epiblast. In a slightly later stage the different connections of the two sets of sense organs is conclusively shewn by the fact that, on the separation of the central nervous system from the epiblast, the optic vesicles remain attached to the former, while the auditory and olfactory vesicles are continuous with the latter. After its separation from the central nervous system the remainder of the epiblast gives rise to the skin, etc., and most probably the epidermic stratum develops into the outer layer of the epidermis and the nervous stratum into the mucous layer. The parts of the organs of special sense, which arise from the epiblast, are developed from the nervous layer. In the Trout (Oellacher, No. 72) both layers are continued over the yolk-sack; but in Clupeus and Gasterosteus only the epidermic has this extension. According to Götte the distinction between the two layers becomes lost in the later embryonic stages.