. Cunningham's Text-book of anatomy. Anatomy. THE EMBRYONIC AEEA. 23 it takes part, as will be seen later, in the formation of the umbilical cord, which connects the foetus with the placenta. The Differentiation of the Embryonic Area.—As the embryonic area is the area of contact between the ecto-mesodermal and the entodermal vesicles it is, at first, circular in outline. As growth continues the area becomes oval, and a linear streak, the primitive streak, appears in that part of the oval which becomes the posterior part of the area (Fig. 31). At the same time the position of the mesodermal ele
. Cunningham's Text-book of anatomy. Anatomy. THE EMBRYONIC AEEA. 23 it takes part, as will be seen later, in the formation of the umbilical cord, which connects the foetus with the placenta. The Differentiation of the Embryonic Area.—As the embryonic area is the area of contact between the ecto-mesodermal and the entodermal vesicles it is, at first, circular in outline. As growth continues the area becomes oval, and a linear streak, the primitive streak, appears in that part of the oval which becomes the posterior part of the area (Fig. 31). At the same time the position of the mesodermal elements of the wall of the ecto-mesodermal vesicle is revealed, for the primitive streak is a thickened ridge of cells which grows from the ecto-mesoderm and projects against the entoderm in the posterior part of the embryonic area, pushing aside the primitive mesoderm which intervened between the adjacent parts of the walls of the ecto-mesodermal and the entodermal vesicles. The deeper cells of the ridge, those next the entoderm, are the mesodermal elements of the primitive ecto-mesoderm, and, by proliferation, they form the larger part, if not the whole, of the embryonic mesoderm and also an organ, called the notochord. The mesoderm produced from the primitive streak may be termed the secondary mesoderm. Immediately after the formation of the primitive streak a groove, the neural groove, appears in the anterior Embryonic area C Plasmodial trophoblast cion-' Cellular trophoblast- ^^j ^ Mesoderm lining I c of trophoblast Mesoderm of amnior Neural fold~- Ectoderm of amnion Neurenteric canal-. Primitive streak - Fig. 31.—Schema of Dorsal Surface of Embryonic Area of Zygote after the removal of part of the Chorion and part of the Amnion. part of the embryonic area. It is formed by the longi- tudinal folding of a thickened plate of ectoderm, the neural plate, which is the rudiment of the whole of the central and peripheral portions of the nervous system, except per- haps the olfa
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectanatomy, bookyear1914
