Archive image from page 170 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 FROM TWELVE TO THIRTY-SIX SOMITES 151 primary fore-brain; but, owing to the simultaneous lateral expan- sion of the telencephalon, the line of separation in the lateral walls forms a curve with the convexity directed posteriorly (Figs. 83 E and 86). (c) The next stage in the differentiation of the telencephalon (20 s to 36 s) is characterized by a rapid expansion and evagina- tion of its lateral walls, while the entire median strip exten
Archive image from page 170 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 FROM TWELVE TO THIRTY-SIX SOMITES 151 primary fore-brain; but, owing to the simultaneous lateral expan- sion of the telencephalon, the line of separation in the lateral walls forms a curve with the convexity directed posteriorly (Figs. 83 E and 86). (c) The next stage in the differentiation of the telencephalon (20 s to 36 s) is characterized by a rapid expansion and evagina- tion of its lateral walls, while the entire median strip extending from the velum transversum to the recessus opticus remains prac- FiG. 85. — Optical sagittal section of the head of an embryo of 22-23 s. The heart is represented entire. atrium. Hyp., anterior lobe of the Hypophysis. Inf., Infun- dibulum. Md., Mandibular arch. or. pi., Oral plate. Pr'o. G., Pre- oral gut. Th., First indication of thyroid. T. p., Tuberculum posterius. V. tr., Velum transversum. Other abbreviations as before. tically unaltered, and thus acts like a rigid band stretched over the surface between these two points. The effect of this is to form a pair of outgrowths that soon begin to project dorsally, anteriorly, and posteriorly (Fig. 83 E); these are the primordia of the cerebral hemispheres, the cavities of which thus appear as lateral diverticula of the median cavity of the telencephalon (Fig. 86). The central part of the telencephalon may be called the telencephalon medium, and the lateral outgrowths the hemi- spheres. The walls of the hemispheres become considerably thicker in this period, but quite uniformly at first, so that the distinction between mantle and basal ganglia is indicated only by position. (See Chap. VIII.)
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