. The development of the chick : an introduction to embryology. Embryology; Chickens -- Embryos. FROM TWELVE TO THIRTY-SIX SOMITES 139 depression bounded in front hy the ectamnion, and on the sides by the amnio-cardiac vesicles of the body-cavity (Fig. 65). The floor of the depression is the proamnion. Just before the for- mation of the head-fold proper, the ectamnion in front of the head becomes irregularly thickened to such an extent as sometimes to present an actually villous surface (Fig. 77; cf. Fig. 67). The head-fold of the amnion begins to form at about the same time as the cephalic fl

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. The development of the chick : an introduction to embryology. Embryology; Chickens -- Embryos. FROM TWELVE TO THIRTY-SIX SOMITES 139 depression bounded in front hy the ectamnion, and on the sides by the amnio-cardiac vesicles of the body-cavity (Fig. 65). The floor of the depression is the proamnion. Just before the for- mation of the head-fold proper, the ectamnion in front of the head becomes irregularly thickened to such an extent as sometimes to present an actually villous surface (Fig. 77; cf. Fig. 67). The head-fold of the amnion begins to form at about the same time as the cephalic flexure. The great expansion of the body-cavity on each side of the head (amnio-cardiac vesicles) causes an elevation of the anterior angle of the ectamnion, and a pocket is formed by fusion of its lateral limbs. This slips over the head of the embryo with aid of the ventral flexure of the head just developing. Inasmuch as the anterior angle of the ectamnion is in the pro- amnion, where there is no mesoderm, and where the ectoderm is in immediate contact with the entoderm, the ento- derm as well as the ectoderm of the pro- amnion is drawn into the head-fold, so that the latter is not at first a fold of the somatopleure. But in the chick the proamniotic part of the head-fold is never very extensive and does not at any topleiire TeItined''"tolorm time extend back of the beginning of the body-wall. ^, . , , . , , .^ . -. B. Ammogenous soma- the mid-bram. Moreover, it is soon in- topleure. C. Choriogenous soma-. FiG. 76. — Entire embryo of 13 s, to show the rela- tions of the ectamnion. a. c, Inner margin of amnio - cardiac vesicles, e. a., Ectamnion. A. Region of the soma- vaded (Fig. 75) by the body-cavity, and topleure. then the entoderm is withdrawn and becomes part of the general splanchnopleure. The proamnion ventral to the head is not invaded by mesoderm until a much later period. The ectodermal thickening marking the junction of amniotic and chorionic somatopleure exten