Archive image from page 159 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 140 THE DEVELOPMENT OF THE CHICK fusion of the ectodermal thickenings of the opposite sides, beginning at the posterior angle of the head-fold and proceeding backwards. The energy of fusion is sufficient in itself to lift the somatopleure up in the form of a fold around the body of the embryo. Thus new parts of the ectodermal thickening are con- stantly being brought together and the fusion progresses steadily, and this in its turn prolo
Archive image from page 159 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 140 THE DEVELOPMENT OF THE CHICK fusion of the ectodermal thickenings of the opposite sides, beginning at the posterior angle of the head-fold and proceeding backwards. The energy of fusion is sufficient in itself to lift the somatopleure up in the form of a fold around the body of the embryo. Thus new parts of the ectodermal thickening are con- stantly being brought together and the fusion progresses steadily, and this in its turn prolongs the lateral amniotic folds. These possess no independent power of elevation of any considerable amount, for, when the initial fold of one side is destroyed by cauterization, the fold of the opposite side remains as an insig- nificant elevation in the somatopleure a long distance lateral to the embryo. Fig. 77. ââ Transverse section through the anterior angle of the ectamnion a few sections in front of the tip of the head. Stage of 14-15 s. b. c. Extra-embryonic body-cavity, c, Cavity in the entoderm, e. a., Ectamnion. The tail-fold arises in an analogous manner to the head-fold, except that there is no proamnion here. The progress of the various folds and their final fusion follows from what has already been said. Practically all of the somatopleure of the pellucid area is amniogenous with the exception, naturally, of that part internal to the limiting sulci that forms the body-wall. What effect has the turning of the embryo on its left side on the amniogenous somatopleure? We will suppose that the latter is primitivelv of equal width on both sides and that the notochord represents approximately the axis of rotation. During the process of rota- tion, the embryo sinks and the lateral limiting sulci become deeper. A direct consequence of the rotation must be, therefore, a strong tension on the somatopleure belonging to the under (left) side, a-b, and practically none on
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