. The Journal of comparative neurology and psychology. ill 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 somew
. The Journal of comparative neurology and psychology. ill 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 posterius. From now on the mammillary recess is alwaysclear. Between it and the primitive optic groove the hypophysislies against the under surface of the primitive inferior lobe. Insome 6 mm. embryos there is to be seen just behind the tip of the. Fig. 3G. Pig embryo, 9 umi., median sagittal section. X 20. hypophysis in sagittal section a slight, but definite, depression andthickening in the brain floor, the beginning of the evagination of theneural part of the hypophysis. This shows a variable developmentin embryos between 6 mm. and 9 mm., but is always clearly present in9 mm. embryos (Fig. 36). After this time the sac grows out andenwraps the tip of the hypophysis and the relations familiar to allembryologists are established. The outgrowth of this sac is the ear-liest mark of the position of the infundibulum and this is a consid-erable distance behind the primitive optic groove, as in selachiansand amphibians. In later development the floor of the primitive Johnston, Forchrain Vesicle in Vertebrates. 499 inferior lobe becomes dra^vni down in funnel shape and the lateralportions become thickened as the tuber cinereum. It is one of thepeculiarities of the mammalian and hum
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