. The development of the chick; an introduction to embryology . rly (Fig. 83 E); these are the primordiaof the cerebral hemisi:)heres, the cavities of which thus appearas lateral diverticula of the median cavity of the telencephalon(Fig. 86). The central part of the telencephalon may be calledthe telencephalon medium, and the lateral outgrowths the hemi-spheres. The walls of the hemispheres become considerablythicker in this period, but quite uniformly at first, so that thedistinction between mantle and basal ganglia is indicated onlyby position. (See Chap. VIII.) 152 THE DEVELOPMENT OF THE CH
. The development of the chick; an introduction to embryology . rly (Fig. 83 E); these are the primordiaof the cerebral hemisi:)heres, the cavities of which thus appearas lateral diverticula of the median cavity of the telencephalon(Fig. 86). The central part of the telencephalon may be calledthe telencephalon medium, and the lateral outgrowths the hemi-spheres. The walls of the hemispheres become considerablythicker in this period, but quite uniformly at first, so that thedistinction between mantle and basal ganglia is indicated onlyby position. (See Chap. VIII.) 152 THE DEVELOPMENT OF THE CHICK The median strip includes the tela choroidea, beginning atthe diencephalon, and the lamina terminalis, which ends at therecessus opticus. These divisions are of great prospective signifi-cance, though at the stage of 36 s they are but slightly differen-tiated, save by their position. A slight thickening of the laminaterminalis just in front of the recessus opticus marks the site ofthe future anterior commissure (Figs. 87 and 88). Metenc. J Mesenc MijeJenc. Te/enc Fig. 86. — Inner view of the brain of a chick of al^oiit 82 hours, drawn froma opt., Chiasma opticus. Ep., Epiphysis (pineal gland). Isth., Isth-mus. v., PHca encephah ventrahs. Rec. opt., Recessus tr., Velum Transversum. Other abbreviations as before. The Diencephalon. The portion of the primary fore-brain pos-terior to the telencephalon is known as the diencephalon. It in-cludes the second and third neuromeres and probably also theventral zones and floor of the first (Fig. 83). A slight constrictionin the roof that appears about the 18 to 20 s stage near the junc-tion of the middle and last third may represent the boundary be-tween the second and third neuromeres; this persists for a longtime and may be traced in the lateral walls to the region of the FROM TWELVE TO THIRTY-SIX SOMITES 153 infimdibiilum (Fig. 83 E); thus the diencephalon may be dividedinto an anterior and poster
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