. The Journal of comparative neurology and psychology. rd, solid con-nection of brain with ectoderm instead of open neuropore, compact-ness of structure) are not favorable for the study of the anterior 464 Journal of Comparative Neurology and Psychology. end of the brain, and the discussions of v. Kupffer, Koltzoff andothers do not seem to the writer to offer anything of value in thepresent state of our knowledge. (See note, p. 535.) It should be noted that the ridge in the brain floor which containsthe optic chiasma* and other decussating tracts is especially high andprominent in cyclostomes
. The Journal of comparative neurology and psychology. rd, solid con-nection of brain with ectoderm instead of open neuropore, compact-ness of structure) are not favorable for the study of the anterior 464 Journal of Comparative Neurology and Psychology. end of the brain, and the discussions of v. Kupffer, Koltzoff andothers do not seem to the writer to offer anything of value in thepresent state of our knowledge. (See note, p. 535.) It should be noted that the ridge in the brain floor which containsthe optic chiasma* and other decussating tracts is especially high andprominent in cyclostomes and that the pit behind the ridge, recessuspostopticus, is quite as deep and sharply marked as that in front ofthe ridge, recessus prceopticus (commonly called recessus opticus). 2. on Head the adult selachian brain the diencephalon presents no peculiar-ities of especial importance to our problems. The limit between the Nucleus habeaulae^ Tectum mesencephaUVelum transversum Epiphysis i Cerebellum ParaphysisTelencephalon. Com. infimaLobus visceralis Rec. praeop. Lobus inferior Optic chiasma Fig. 1. The mesial surface of the right half of the brain of Johnston, 1906. For the significance of the abbreviations used on all ofthe figures, see the list at the end of this article. diencephalon and telencephalon is clearly marked dorsally by a deepvelum transversum which forms the anterior wall of the dorsal sacbelonging to the diencephalon, and the posterior wall of the para-physis belonging to the telencephalon (Fig. 1). In front of the para-physis the membraneous roof is complexly infolded to form the plexus Johnston, Forehrain Vesicle in Vertebrates. 465 chorioideus of the telencephalon. The anterior limb of the plexusis attached to the massive nervous wall which overarches the frontpart of the median ventricle. In the more primitive selachians suchas Heptanchus and in Chimsera the massive roof is smaller and themembraneous ro
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