. The Journal of comparative neurology and psychology. complete transverse segment of the brain, asHis always insisted, there is no alternative but to include the opticchiasma within it. The primitive optic groove which bounds theoptic chiasma behind belongs to the same neuromere with the opticvesicles and therefore is included in the diencephalon. The telen-cephalon can include no more than the optic chiasma and the associ-ated decussations in the brain floor which lie in the terminal boundary Ix^tween the diencephalon and telencephalon is markedby the velum transversum above and by
. The Journal of comparative neurology and psychology. complete transverse segment of the brain, asHis always insisted, there is no alternative but to include the opticchiasma within it. The primitive optic groove which bounds theoptic chiasma behind belongs to the same neuromere with the opticvesicles and therefore is included in the diencephalon. The telen-cephalon can include no more than the optic chiasma and the associ-ated decussations in the brain floor which lie in the terminal boundary Ix^tween the diencephalon and telencephalon is markedby the velum transversum above and by the primitive optic groove orpostoptic recess below. In adult mammals, in which both these land- 510 Joiirnnl of Comparative Neurology and Psychology. marks have disappeared, the boundary can be defined by a line dra\vnjnst behind the interventriciihir foramen and meeting the posteriorsurface of the chiasma ridge (Fig. 44). The external groove which separates the diencephalon and telen-cephalon is usually well marked and in the brains of amphibians,. Fig. 43. Sketches to illustrate the boundary line between the diencephalonand the telencephalon. The brains of a selachian (A) and an amphibian(B) are outlined as seen from the medial surface and the boundary set byHis is indicated by a dotted line, that fixed in this paper by a heavy con-tinuous line. reptiles and mammals increases in depth and prominence with theenlargement of the cerebral hemispheres. The description of theearly development has shown that the lateral chorioid plexus isformed in manunals immediately in front of the velum transversumand of the groove which continues from the velum transversumaround the lateral wall of the brain. From this it results that in the Johnston, Forrbrain Vesicle in Vertebrates. 511 adult the chorioid fissure is found at the bottom of the very deepgroove between the hemispheres and the brain stem and that theboundary between diencephalon and telencephalon runs just alongthe posterior (th
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