. The Journal of comparative neurology and psychology. thecavity of the stalk is seen near the base of the brain. It is notice-able that this cavity no longer communicates directly with the primi-tive optic groove. It seems equally closely related to the terminalpit. Between the two pits the brain wall is somewhat thickened. Johnston, Forcbrain Vesicle ni Vertebrates. 481 This thickening corresponds to the anterior border of the neural plateand the lower border of the neuropore. It is the terminal ridgeof the early embryo with open neural plate, Figs. 3, 4, 5, 6. Fromthis terminal thickening a
. The Journal of comparative neurology and psychology. thecavity of the stalk is seen near the base of the brain. It is notice-able that this cavity no longer communicates directly with the primi-tive optic groove. It seems equally closely related to the terminalpit. Between the two pits the brain wall is somewhat thickened. Johnston, Forcbrain Vesicle ni Vertebrates. 481 This thickening corresponds to the anterior border of the neural plateand the lower border of the neuropore. It is the terminal ridgeof the early embryo with open neural plate, Figs. 3, 4, 5, 6. Fromthis terminal thickening a ridge is seen in Fig. 16 running obliquelycaudo-laterad behind the optic vesicle and cutting it off from commu-nication with the primitive optic groove. This is the beginning ofa change in the relations of the optic vesicle which the followingstages show completed. An embryo of 42 somites is shown in Fig. 17 drawn from a modelof the right half of the head and a corresponding model of anembryo of Balfours stage K is sho^\Ti in Fig. 18. In these it is. Fig. 17. Squalus ac, 42 somites, medial aspect of a model of the right halfof the head. This is the same embryo as the one shown in Fig. 15. clear that the optic vesicle is no longer connected with the primitiveoptic groove, but the two vesicles are now connected with one anotherby the terminal pit at the lower border of the lamina is the pit which remains connected with the hollow optic stalkas long as that persists and is known in the adult as the recessusopticus (His) ; better called recessus praopticus. 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
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