. The Journal of comparative neurology and psychology. ni ec Fig. 14. Section from same series as Fig. 13, passing tbrougli tlie tip of theanterior head cavity on the left side and in front of this cavity on the rightside. X Fig. 15. Squalus ac, 42 somites. Left view of a model of the brain, meso-derm, ganglia and mesectoderm derived from the neural crest. The auditorypit with the ectoderm bordering it are also shown, x ^0. The boundaries ofthe mesoderm are marked in black lines. 3, //, 5, somites of Van Wijhesseries. Johnston, Forchrain Vesicle in Vcrtchrotes. 479 Developiiiciit of the
. The Journal of comparative neurology and psychology. ni ec Fig. 14. Section from same series as Fig. 13, passing tbrougli tlie tip of theanterior head cavity on the left side and in front of this cavity on the rightside. X Fig. 15. Squalus ac, 42 somites. Left view of a model of the brain, meso-derm, ganglia and mesectoderm derived from the neural crest. The auditorypit with the ectoderm bordering it are also shown, x ^0. The boundaries ofthe mesoderm are marked in black lines. 3, //, 5, somites of Van Wijhesseries. Johnston, Forchrain Vesicle in Vcrtchrotes. 479 Developiiiciit of the Forebrain. a. The Optic Vesicle and the Primitive Optic Groove.—The earlyappearance of the optic vesicles on the open neural plate has beendescribed by Locy (1895), and the writer has elsewhere (1905,1906, 1909) brought together the evidence that the optic vesicle isderived from the alar plate of the embryonic neural tube. Thestudy of its relations for the present purpose begins with embryosof about 15 somites. The medial surface of the right half of amodel of the head of such an embryo is shown in Fig. 5. The neuraltube has a cephalic flexure and is open at the neuropore and for ashort distance along the dorsal surface.
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