. The Journal of comparative neurology and psychology. 8). In front of the epiphysis and in the same archappears the commissura hahenularis. This arch then is the roof ofthe diencephalon. It is bounded in front in all other classes ofvertebrates by the velum transversum. The fold to which this namehas been given lies in the proper segmental position. This is furthersupported by its relations to the structures in front of it. Immedi-ately in front of the velum the roof is raised in a distinct arch. 500 Journal of Comparative Neurology and Psychology. This corresponds in position to the paraphys
. The Journal of comparative neurology and psychology. 8). In front of the epiphysis and in the same archappears the commissura hahenularis. This arch then is the roof ofthe diencephalon. It is bounded in front in all other classes ofvertebrates by the velum transversum. The fold to which this namehas been given lies in the proper segmental position. This is furthersupported by its relations to the structures in front of it. Immedi-ately in front of the velum the roof is raised in a distinct arch. 500 Journal of Comparative Neurology and Psychology. This corresponds in position to the paraphysis of lower forms ( 30 to 33). Since there is no glandular development knownin mammals, it may be called the paraphysal arch, a name whichMinot (1901) applies to the corresponding structure in birds. Infront of the paraphysal arch a membrane continues forward to meetthe lamina terminalis. ^Vhen the lateral cerebral vesicles are formedit is seen (Figs, 37, 38, 39) that this membrane lies over the ventriclebetween the interventricular Fig. 38. Pig embryo, 15 mm. Median sagittal section of the dotted outline of the hemisphere is reconstructed 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 th
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