Archive image from page 161 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 142 THE DEVELOPMENT OF THE CHICK inevitably be thrown into secondary folds by the approximation of points c and d (Fig. 78 C). Study of the fusion of the amniotic folds in actual sections shows, that the line of fusion of the opposite amniotic limbs is over the dorsal surface of the embryo only so long as the latter lies flat on the yolk; it does not follow the turning of the embryo on to its left side, and the consequence is that, after
Archive image from page 161 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 142 THE DEVELOPMENT OF THE CHICK inevitably be thrown into secondary folds by the approximation of points c and d (Fig. 78 C). Study of the fusion of the amniotic folds in actual sections shows, that the line of fusion of the opposite amniotic limbs is over the dorsal surface of the embryo only so long as the latter lies flat on the yolk; it does not follow the turning of the embryo on to its left side, and the consequence is that, after rotation of the embryo, the line of fusion lies over the upper (right) side of the embryo, often opposite the horizontal level of the intestine (Fig. 79). Thus one fold of the amnion passes all the way from the Tinder side over the back of the embryo and around on the other side to the line of fusion, and thus is several times as long as the opposite limb. Moreover, the amniotic fold of the right Fig. 79. — Section of an embryo of about 60 hours to show the sec- ondary fold (s. f.) of the amnion on the right side. e. a., Ectamnion. s. f., Secondary fold. 1., Left, r., Right. side is invariably thicker than that of the left side, and is always thrown into secondary folds at the place of turning (Fig. 79). These conditions are satisfactorily explained, as noted above, by the mere turning of the embryo on its side. One must therefore distinguish in the upper limb of the am- nion two kinds of folds: (1) The ordinary amniotic fold induced by the fusion of the right and left folds, and (2) secondary folds formed simply by the process of twisting of the embryo. These secondary folds of the amnion are very transitory, except in two regions: (1) Above the hind end of the heart (apex of ventricle), and continuing a short distance behind it; (2) in the region immediately in front of the allantois, at sixty to seventy hours, thus in the neighborhood of the final closure of the amniot
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