Archive image from page 169 of The development of the chick;. The development of the chick; an introduction to embryology . developmentofchi00lill Year: 1908 150 THE DEVELOPMENT OF THE CHICK diameter of the fore-brain itself, though this is relatively slight up to twenty somites, but it enhances the general effect of the change in position of the optic stalk. The subsequent history of the optic vesicles is given beyond. (h) The deUmitation of the tel- and diencephalon is initiated by a forward expansion of the anterior end of the primary fore- brain, which becomes the telencephalon or seconda
Archive image from page 169 of The development of the chick;. The development of the chick; an introduction to embryology . developmentofchi00lill Year: 1908 150 THE DEVELOPMENT OF THE CHICK diameter of the fore-brain itself, though this is relatively slight up to twenty somites, but it enhances the general effect of the change in position of the optic stalk. The subsequent history of the optic vesicles is given beyond. (h) The deUmitation of the tel- and diencephalon is initiated by a forward expansion of the anterior end of the primary fore- brain, which becomes the telencephalon or secondary fore-brain, the remainder being then known as the diencephalon or 'tween brain. The expansion proceeds very rapidly from the 14 s stage, and it is probable that it involves only the dorsal zones. It is, Ectdw £ :-:M yfm§ O:... .. > â - â ' '\kj&i- '4, \>, Fig. 84. â Transverse section through the fore-brain and optic vesicles of a 16-s embryo. Am. F., Amniotic fold. Ectam., Ectamnion. L., Left side, , Optic stalk. R., Right side. Other abbreviations as before. however, difficult to establish an exact line of demarcation be- tween the two subdivisions of the primary fore-brain, until about the 18 to 20 s stage, when a slight transverse fold or indentation in the roof (velum transversum) gives a dorsal landmark (Figs. 73, 85); the recessus opticus forms the ventral boundary between the two. The velum transversum lies a considerable distance above the dorsal end of the lamina terminalis, but it is difficult to say just how far, owing to the inclefiniteness of this point for some time after the disappearance of the neuropore. A line drawn between the velum transversum and the recessus opticus mav be taken as the boundary between the two divisions of the
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