. A laboratory manual and text-book of embryology. Embryology. Fig. 252.—Reduction of the bulbo-ventricular fold of the heart (Keith). Ao. aortic bulb; Au, atrium; B, bulbus cordis; RV, right ventricle; LV, left ventricle; P (in b) pulmonary artery. are: (1) the complete division of the atrium and ventricle, each into right and left chambers; (2) the division of the bulb and truncus arteriosus into the aorta and pulmonary artery; (3) the absorption of the sinus venosus into the wall of the right atrium; (4) the development of the semilunar and atrio-ventricular valves. The first of these chang
. A laboratory manual and text-book of embryology. Embryology. Fig. 252.—Reduction of the bulbo-ventricular fold of the heart (Keith). Ao. aortic bulb; Au, atrium; B, bulbus cordis; RV, right ventricle; LV, left ventricle; P (in b) pulmonary artery. are: (1) the complete division of the atrium and ventricle, each into right and left chambers; (2) the division of the bulb and truncus arteriosus into the aorta and pulmonary artery; (3) the absorption of the sinus venosus into the wall of the right atrium; (4) the development of the semilunar and atrio-ventricular valves. The first of these changes is completed only after birth. Endocardial Cushions and Atrial Septa.—In embryos of 5 to 7 mm. there develops a thin sickle-like membrane from the mid-dorsal wall of the atrium (Figs. 253 and 254). This is called the atrial septum primum (I). Simultaneously, endothelial thickenings appear in the dorsal and ventral walls of the atrial canal (Fig. 254, A, B). These are the endocardial cushions which later fuse, thus divid- ing the single atrial canal into right and left atrio-ventricular canals. The atrium is now partly divided into right and left atria which, however, communicate ven-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Prentiss, Charles William, 1874-1915. Philadelphia, London, W. B. Saunders
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