Archive image from page 112 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 HEAD-FOLD TO TWELVE SOMITES 93 II. Formation of the Fore-gut The extension of the amnio-cardiac vesicles between the ectodermal and entodermal layers of the head-fold introduces a section of the body-cavity (pericardium) between these layers and at the same time converts the ectodermal limb into a portion of the somatopleure, and the entodermal limb into a portion of the splanchnopleure. (See p. 115.) The splanchnopleuric head-fold exten
Archive image from page 112 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 HEAD-FOLD TO TWELVE SOMITES 93 II. Formation of the Fore-gut The extension of the amnio-cardiac vesicles between the ectodermal and entodermal layers of the head-fold introduces a section of the body-cavity (pericardium) between these layers and at the same time converts the ectodermal limb into a portion of the somatopleure, and the entodermal limb into a portion of the splanchnopleure. (See p. 115.) The splanchnopleuric head-fold extends posteriorly very rapidly after the invasion of the body-cavity, while the somatopleuric fold apparently remains fixed for some time, though the head-fold appears to Fig. 47. — Head-fold region of Fig. 46 highly magnified. For abbreviations see Fig. 46. become deeper, owing to the forward extension of the head above the blastoderm. The posterior extension of the splanch- nopleuric head-fold lengthens the floor of the fore-gut; it is caused by the median growth and concrescence of folds of the splanchnopleure (Fig. 53). Along with this process is involved the development of the heart described farther on. The growth in length of the fore-gut may be realized by a comparison of Figs. 50, 52, 62, etc. Thus by the 12 s stage a considerable section of the fore-gut is already established (Fig. 63); this is the pharyngeal division: from the first it is extremely broad, and lunate in cross-section (Fig. 54), the floor being composed of columnar cells, and the roof
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