. Human embryology and morphology. Embryology, Human; Morphology. DEVELOPMENT OF THE OVUM. 89 which spreads on the inner aspect of Eauber's layer, and absorbs or mixes with it, to form one layer of more or less columnar cells, the epiblast or ectoderm (Fig. 67); an inner layer also spreads out from the germinal group, and forms a second lining to the blastodermic vesicle, the hypoblast or endoderm (Fig. 67). The Primitive Streak and Groove.—In the diagrammatic section of the bilaminar blastoderm, given in figure 67, the hypo- blast and epiblast are seen to be fused at one point, and at the poi
. Human embryology and morphology. Embryology, Human; Morphology. DEVELOPMENT OF THE OVUM. 89 which spreads on the inner aspect of Eauber's layer, and absorbs or mixes with it, to form one layer of more or less columnar cells, the epiblast or ectoderm (Fig. 67); an inner layer also spreads out from the germinal group, and forms a second lining to the blastodermic vesicle, the hypoblast or endoderm (Fig. 67). The Primitive Streak and Groove.—In the diagrammatic section of the bilaminar blastoderm, given in figure 67, the hypo- blast and epiblast are seen to be fused at one point, and at the point of fusion the epiblast is thickened and somewhat depressed. When the blastodermic vesicle is viewed from the surface, the line along which the fusion of the two layers and thickening of the epiblast take place is marked by the primitive streak and groove (Fig. 68). At the anterior end of the groove is situated the blastopore or neurenteric canal, an opening into the cavity which the hypoblast encloses—the archenteron (see Figs. 67 and 75). Eound the blastopore and along the primitive streak the epiblast is continuous with the hypoblast. The Mesoblast.—The mesoblast or mesoderm, the third of the primitive blastodermic layers, is produced from the margins of the primitive streak (Fig. 67). At the line of junction of the. Fig. edullary fold medullary groove blastopore primitive groove \& streak 3.—Diagram of the Embryogenie area of a Bilaminar Blastoderm viewed from above. epiblast and hypoblast there is a free proliferation of cells which spreads out between the two primitive layers, and gives rise to a third or intermediate layer—the mesoblast (Fig. 69). In lower vertebrates the mesoblast is entirely produced from the hypoblast. Neural Canal.—It is at this early stage, while the mesoblastic cells swarm in between the epiblast and hypoblast, and while the. Please note that these images are extracted from scanned page images that may have been digitally enhanced
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