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Amnion 
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Embnromc 
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Primitive 
Streak 
Figure 9A-C: Schematic drawings of the embryonic disc and its associated 
extraembryonic membranes during the third week. A: the amniotic cavity has been 
opened to show the primitive streak, a midline thickening of the epiblast. Part of the 
yolk sac has been cut away to show the bilaminar embryonic disc (epiblast and 
hypoblast). The transverse section {lower right of A) illustrates the proliferation and 
migration of cells from the primitive streak to form embryonic mesoderm. B and C: 
drawings illustrating early formation of the notochordal process from the primitive 
knot of the primitive streak. In the longitudinal sections on the right side, note that the 
notochordal process grows cranially in the median plane between the embryonic 
ectoderm and endoderm. (Reprinted from Moore, Essentials of Human Embryology, 
1988, Fig. 3-1, page 17, with permission from Elsevier.] 
PRE-PUBLICATION VERSION 
