The development of the human body; a manual of human embryology . osethe entire ovum, develops in situ during the stages imme-diately succeeding segmentation, and the extra-embry-onic mesoderm, instead of growing out from the embryoto ejticlose the yolk-sac, splits off directly from the envel-oping layer. The earliest stages in the development ofthe amnion are not yet known for the human embryo, butfrom the condition in which it is found in the Petersembryo (Fig. 35) and in the embryo of von Spee(Fig. 37) it is probable that it arises, not by the fusion ofthe edges of a fold, as in the ch
The development of the human body; a manual of human embryology . osethe entire ovum, develops in situ during the stages imme-diately succeeding segmentation, and the extra-embry-onic mesoderm, instead of growing out from the embryoto ejticlose the yolk-sac, splits off directly from the envel-oping layer. The earliest stages in the development ofthe amnion are not yet known for the human embryo, butfrom the condition in which it is found in the Petersembryo (Fig. 35) and in the embryo of von Spee(Fig. 37) it is probable that it arises, not by the fusion ofthe edges of a fold, as in the chick, but by a vacuolizationof a portion of the inner cell-mass, as has been describedas occurring in the bat (p. 71). It is, then, a closedcavity from the very beginning, the floor of the cavity 132 THE DEVELOPMENT OF THE HUMAN BODY. being formed by the embryonic disk, its posterior wall bythe anterior surface of the belly-stalk, while its roof andsides are thin and composed of a single layer of flattenedectodermal cells lined on the outside by a layer of meso-. Fig. 63.—Diagrams Illustrating the Formation of the Umbilical Cord. Tlie heavy black line represents the embryonic ectoderm; the dottedline represents the line of reflexion of the body ectoderm into that ofthe amnion. Ac, Amniotic cavity; Al, allantois; Be, extra-embry-onic coelom; Bs, belly-stalk; Ch, chorion; P, placenta; Uc, umbilicalcord; V, chorionic villi; Ys, yolk-sac. derm continuous with the somatic mesoderm of the em-bryo and the mesoderm of the belly-stalk (Fig. 63, A). When the bending downward of the peripheral portionsof the embryonic disk to close in the ventral surface of the THE AMNION. I 33 embryo occurs, the line of attachment of the amnion tothe disk is also carried ventrally (Fig. 63, B), so that whenthe constriction off of the embryo is practically completed,the amnion is attached anteriorly to the margin of theumbilicus and posteriorly to the extremity of the band ofectoderm lining what may now
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
