At the end of the fourth week, two mesenchymal cushions, the atrioventric-ular endocardial cushions, appear at the superior and inferior borders of the atrioventricular canal (Figs. 11.16 and 11.17). Initially the atrioventricular canal gives access only to the primitive left ventricle and is separated from the bul-bus cordis by the bulbo(cono)ventricular flange (Fig. 11.8). Near the end of the fifth week, however, the posterior extremity of the flange terminates almost midway along the base of the superior endocardial cushion and is much less prominent than before (Fig. 11.17). Since the atrioventricular canal enlarges to the right, blood passing through the atrioventricular orifice now has direct access to the primitive left as well as the primitive right ventricle.
In addition to the superior and inferior endocardial cushions, the two lateral atrioventricular cushions appear on the right and left borders of the canal (Figs. 11.16 and 11.17). The superior and inferior cushions, in the meantime, project further into the lumen and fuse, resulting in a complete division of the canal into right and left atrioventricular orifices by the end of the fifth week (Fig. 11.16).
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