. The development of the chick; an introduction to embryology . -YojTh Fig. 74. — Head of the same embryofrom the ventral side. Abbreviationsas before. fifP /s/A. jri(._ 75_ — Median sagittal section of the head of an embryo of IS s. H. F. Am., Head-fold of the amnion. Ph., Pharynx. Isth., Region ofthe isthmus, pro. g., Preoral gut. or. pi., Oral plate. Rec. opt., Recessusopticus. S. Sinus venosus. Other abbreviations as before. immediately turns backwards over the head, forming a completecap (Figs. 67, 71, 75, etc.); the side limbs of the head-fold arethen elongated backwards, and
. The development of the chick; an introduction to embryology . -YojTh Fig. 74. — Head of the same embryofrom the ventral side. Abbreviationsas before. fifP /s/A. jri(._ 75_ — Median sagittal section of the head of an embryo of IS s. H. F. Am., Head-fold of the amnion. Ph., Pharynx. Isth., Region ofthe isthmus, pro. g., Preoral gut. or. pi., Oral plate. Rec. opt., Recessusopticus. S. Sinus venosus. Other abbreviations as before. immediately turns backwards over the head, forming a completecap (Figs. 67, 71, 75, etc.); the side limbs of the head-fold arethen elongated backwards, and are here known as the lateralfolds of the amnion; these rise up and arch over the eml)ryo 138 THE DEVELOPMENT OF THE CHICK (Figs. 109 nnd 110). In each fold one can distinfiuish an amnioticor internal limb, and a chorionic or external limi) meeting at ornear the angle of the folds, the line of junction being markedby an ectodermal thickening, the ectamnion. Fusion of theright and left lateral folds begins at the head-cap, and progressesi)ack\vards in such a way tiiat the right and left amniotic limbsbecome continuous with one another, similarly the righ
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