. The development of the chick; an introduction to embryology . is no mesoderm,and where the ectoderm is in immediatecontact with the entoderm, the ento-derm as well as the ectoderm of the pro-amnion is drawn into the head-fold, sothat the latter is not at first a fold ofthe somatopleure. But in the chick the proamniotic part of the head-fold is , 1 , , A. Region of the soma- never very extensive and does not at any topleure destined to form time extend back of the beginning of the body-wall. ^1 .... ,, . . B. Amniosrenous soma- the mid-bram. Moreover, it is soon in- topleure. vaded (Fig. 75)
. The development of the chick; an introduction to embryology . is no mesoderm,and where the ectoderm is in immediatecontact with the entoderm, the ento-derm as well as the ectoderm of the pro-amnion is drawn into the head-fold, sothat the latter is not at first a fold ofthe somatopleure. But in the chick the proamniotic part of the head-fold is , 1 , , A. Region of the soma- never very extensive and does not at any topleure destined to form time extend back of the beginning of the body-wall. ^1 .... ,, . . B. Amniosrenous soma- the mid-bram. Moreover, it is soon in- topleure. vaded (Fig. 75) bv the bodv-cavitv, and ^- Choriogenous soma- then the entoderm is withdrawn and becomes part of the general splanchnopleure.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 extends backwards very rapidly and always precedes the origin of folds in any region. The lateral folds themselves appear to owe their origin to the progressive. Fig. 76. — Entire embryoof 13 s, to shoAV the rela-tions of the ectamnion. a. c, Inner margin ofamnio-cardiac vesicles,e. a., Ectamnion. 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 proceedingbackwards. The energy of fusion is sufficient in itself to lift thesomatopleure up in the form of a fold around the body of theembryo. 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. Thesepossess no independent power of elevation of any considerableamount, for, when the initial fold of one side is destroyed bycauterization, the fold of the opposite side remains as an insig-nificant elevation in the somatopleure a long distance lateral tothe embryo.
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Keywords: ., bookauthorlilliefrankrattray187, bookcentury1900, bookdecade1910
