. The development of the chick; an introduction to embryology . e endothe-lium in the floor and roof re-spectively of the auricular canal(cf. Figs. 202, 203 and 204).These cushions rapidly thickenso as to restrict the center ofthe atrioventricular aperture,and finally, fusing together, di-vide the latter into two verti-cally-elongated apertures, rightand left respectively. The timeof formation of this large endo-cardial cushion dividing the au-ricular canal is coincident withthe formation of the other septa. (e) Completion of the Septa. Thus by the end of the fifthor the beginning of the sixth
. The development of the chick; an introduction to embryology . e endothe-lium in the floor and roof re-spectively of the auricular canal(cf. Figs. 202, 203 and 204).These cushions rapidly thickenso as to restrict the center ofthe atrioventricular aperture,and finally, fusing together, di-vide the latter into two verti-cally-elongated apertures, rightand left respectively. The timeof formation of this large endo-cardial cushion dividing the au-ricular canal is coincident withthe formation of the other septa. (e) Completion of the Septa. Thus by the end of the fifthor the beginning of the sixth day of incubation, the heart isprepared for the rapid completion of a double circulation. Theembryonic circulation is never completely double, however, forthe reason that the embryonic resjiiratory organ (allantois)belongs to the aortic system, and full pulmonary circulation doesnot begin until after hatching. However, between the sixthand eighth days the right and left chambers of the heart becomecompletely separated, except that the interauricular foramina. Fig. 203. — Reconstruction of theheart of a chick embryo of , seen from right of the wall of the right auricleis cut away. (After Masius.)B. Co., Bulbus cordis. D. C, Ductof Cuvier. E. C. d., Dorsal andventral endothelial cushions. O. S. of the sinus venosus into theright auricle. ,, Primary andseconthiry ostia or inter-auricular con-nections. 356 THE DEVELOPMENT OF THE CHICK remain until hatchinp;, and serve as a passageway of blood fromthe right side to the left side. The completion of the caidiac septa takes place in such away that the aorta becomes connected with the left ventricle,the pulmonary artery remaining in connection with the understand how this occurs it is necessary to remember that,although the bulbus arteriosus is primitively connected with theright side of the ventricle, the revolution of the latter has trans-ferred the bulbus to the middle line
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