. The Journal of comparative neurology and psychology. In the latter part of Balfours stage K the optic tract fibers beginto appear in the chiasma. This lies immediately behind the terminalpit and in front of the primitive optic groove (Figs. 22, A and B).and therefore lies in the temiinal ridge. The lateral prolongationof this ridge, which has been described as running obliquely across 482 Journal of Comparative Neurology and Psychology. the primitive optic groove (Fig. 16), furnishes a pathway for theoptic tract fibers as they grow in from the retina to the optic centersin the thalamus and t
. The Journal of comparative neurology and psychology. In the latter part of Balfours stage K the optic tract fibers beginto appear in the chiasma. This lies immediately behind the terminalpit and in front of the primitive optic groove (Figs. 22, A and B).and therefore lies in the temiinal ridge. The lateral prolongationof this ridge, which has been described as running obliquely across 482 Journal of Comparative Neurology and Psychology. the primitive optic groove (Fig. 16), furnishes a pathway for theoptic tract fibers as they grow in from the retina to the optic centersin the thalamus and tectum opticum. The ridge may therefore becalled the oijtic ridge. The primitive optic groove from this stageon is to be seen just behind the transverse ridge occupied by the opticchiasma and other decussations. Referring to the general descrip-tion at the beginning of this article it will be seen that what I havecalled the terminal pit in the early embryo is the same as that calledby His the optic recess, and that to which I have in all my previous. Fig. 18. Squalus ac, late stage K of Balfour, medial aspect of a model ofthe right half of the head, x 25. papers given the name of preoptic recess. The primitive opticgroove I have heretofore called the postoptic recess. These termshave been used by some other authors (Mrs. Gage, Sterzi, and others),but not by all. I wish to emphasize the necessity of recognizingthe two pits and applying to them clearly distinctive names, becausethey are both related to the optic vesicle and because the postopticrecess has heretofore been confused with the infundibulum. b. Remainder of the Floor of the Diencephalon.—As soon as thehead-bend of the brain tube appears, a broad depression of the floorof the forebrain vesicle can be seen which corresponds to the future Johnston, Forebrain Vesicle in Vertebrates. 483 inferior lobes (Fig. 16). The anterior part of this is the relativelydeep and sharply marked primitive optic groove. The posteriorlx)iind
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