. The Journal of comparative neurology and psychology. de (Fig. 34). Fromthe figure it will be seen that this brain agrees very closely withthat of the Squalus embryo of about 20 somites. The cavity of theoptic stalk is continuous with a groove which traverses the medianline, the primitive optic groove. Behind this is the primitive inferiorlobe, a ventral expansion bounded caudally by the tuberculum pos-terius. In front of the primitive optic groove is a transverse ridgewhose cross section in the median plane presents the form of an arch. Johnston, Forebrain Vesicle in Vertebrates. 497 This is
. The Journal of comparative neurology and psychology. de (Fig. 34). Fromthe figure it will be seen that this brain agrees very closely withthat of the Squalus embryo of about 20 somites. The cavity of theoptic stalk is continuous with a groove which traverses the medianline, the primitive optic groove. Behind this is the primitive inferiorlobe, a ventral expansion bounded caudally by the tuberculum pos-terius. In front of the primitive optic groove is a transverse ridgewhose cross section in the median plane presents the form of an arch. Johnston, Forebrain Vesicle in Vertebrates. 497 This is the terminal ridge. In front of this is a median pit, theterminal pit. Following the lamina terminalis around the front endof the brain there is found in the dorsal wall a distinct transversefold, followed by an arched portion and a second more shallow this fold and for some distance behind it are seen in later embryosthe fibers of the posterior commissure and the decussation of thetectum mesencephali. The more anterior and deeper of the two. Fig. 35. Pig embryo, 6 mm. A., median sagittal section. B and C, para-sagittal sections. Neuromeres numbered in Roman, x 20. folds is the velum transversum, as the further description will makeclear. In embryos of 6 and 7 mm. the same change of relations inthe optic region takes place as has been described above for Squalusand Amblystoma. The terminal ridge becomes prolonged caudo-laterad as the optic ridge and in these later appear the optic chiasmaand tracts. The identity of these structures in the classes of animalsstudied is absolutely clear. 498 Journal of Comparative Neurology and Psychology. The time of development of the recesses and of theneural part of the hypophysis (saccus vasculosus) varies somewhatin pig embryos. In most 6 mm. embryos the mammillary recess isalready clearly recognizable as a caudal expansion of the primitiveinferior lobe (Fig. 35, A) whose border above and behind is thetviberculum posteri
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