. The Journal of comparative neurology and psychology. nsversum. Behind the optic vesicle areclearly seen in the figure three segments. In connection with thefirst of these (neuromere iii) appears later the epiphysis. The othertwo (iv and v) obviously enter into the mesencephalon. In selach-ians these two segments have connected with them respectively the 508 Journal of Comparative Neurology and Psychology. thalamic nerve of Miss Piatt which probably forms the ciliary gan-glion, and the part of the neural crest which forms the ophthalmicdivision of the trigeminus. The terminal part of the neur
. The Journal of comparative neurology and psychology. nsversum. Behind the optic vesicle areclearly seen in the figure three segments. In connection with thefirst of these (neuromere iii) appears later the epiphysis. The othertwo (iv and v) obviously enter into the mesencephalon. In selach-ians these two segments have connected with them respectively the 508 Journal of Comparative Neurology and Psychology. thalamic nerve of Miss Piatt which probably forms the ciliary gan-glion, and the part of the neural crest which forms the ophthalmicdivision of the trigeminus. The terminal part of the neural crest inclose relation with the neuropore presumably gives rise to theganglion of the nervus terminalis in selachians. If this be true, everyneuromere of the embryonic brain has connected with it in one classof vertebrates or another some sensory nerve or sense organ (includ-ing the optic vesicle and epiphysis. The five brain segments areequally clearly to be seen in Figs. 34 and 35. If. Boundary between Diencephalon and Telenceplialon.—The pos-. FiG. 42. Pig embryo, 7 mm. Two parasagittal sections to show the seg-ments of the forebraiu and mid-brain. Compare figs. 34, 35 and 18. terior boundary of the diencephalon has never been in dispute. Itis the constriction between the forebrain and midbrain vesicles andis later marked dorsally by the posterior commissure and ventrallyby the tuberculum posterius. When the forebrain vesicle becomesdivided into diencephalon and telencephalon the exact location ofthe boundary between them has not been entirely clear. In all verte-brates in which a definite velum transversum is recognizable this isconsidered as the mark of the boundary. The existence of aparaphysis and lateral plexus chorioideus in front of the velum andof a dorsal sac and one or two epiphyses behind it is now so thor- Johnston, Forcbrain Vesicle in Vertebrates. 509 ougiilj understood as to need no further comment (Gaupp 1898,Minot 1901, Johnston 1905, 190G). The velum t
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