. The Journal of comparative neurology and psychology. cted from several 15. As development proceeds the velum transversum becomes a foldwith a sharper angle but less deep in proportion to the size of thebrain. The paraphysal arch remains a distinct median pouch untilthe lateral vesicles are well formed. In sagittal sections to one sideof the median plane the lateral ventricle appears as a dorsal cavityopening by way of the inten^entricular foramen in front of theparaphysal arch into the median ventricle. (Fig. 38, 39.) Thesesimple relations persist up to the 17 mm. stage or later.
. The Journal of comparative neurology and psychology. cted from several 15. As development proceeds the velum transversum becomes a foldwith a sharper angle but less deep in proportion to the size of thebrain. The paraphysal arch remains a distinct median pouch untilthe lateral vesicles are well formed. In sagittal sections to one sideof the median plane the lateral ventricle appears as a dorsal cavityopening by way of the inten^entricular foramen in front of theparaphysal arch into the median ventricle. (Fig. 38, 39.) Thesesimple relations persist up to the 17 mm. stage or later. By the Johnston, Forehrain Vesicle in Vertebrates. 501 15 to 17 mm. embryo the chorioid plexus of the lateral ventricle isforming. Its position with reference to the velum transversum isshown in a parasagittal section of a 17 mm. embryo (Fig. 40) andin three frontal sections of the brain of a 15 mm. embryo (Fig. 41).The velum transversum is not only a dorsal fold, but is continuedaround the lateral wall of the brain as the constriction (external fur-. FiG. 39. Pig embryo, 17 mm. Median sagittal section reconstructed fromseveral sections. The outline of the hemisphere lateral to the median planeis shown in dotted lines, x 12%. row and internal ridge) between the diencephalon and the telen-cephalon. By the stage mentioned the cerebral vesicle is sufficientlyexpanded to push back past the boundary line. In the angle betweenthe vesicle and the diencephalon chorioid plexus pushinginto the lateral ventricle. It appears as a folding of the anteriorwall or limb of the velum transversum and its lateral prolongation. 502 Journal of Comparative Neurology and Psychology. In this way appears the chorioid fissure whose further history neednot be traced. Near the median plane the plexus appears as a foldprojecting into the interventricular foramen and separated from the
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