. The development of the chick; an introduction to embryology. Birds -- Embryology. LATER DEVELOPMENT OF VASCULAR SYSTEM 355 completely along its entire free border with the endothelial cushions of the latter. It would thus establish a complete par- tition between the two auricles were it not for the fact that secondary perforations arise in it before its free edge meets the endothelial cushions (Fig. 203), logical significance as the fora- men ovale in the mammalian heart, and persist through the period of incubation, closing soon after hatching. (d) The Cushion-septum (Sep- tum of the Auricu
. The development of the chick; an introduction to embryology. Birds -- Embryology. LATER DEVELOPMENT OF VASCULAR SYSTEM 355 completely along its entire free border with the endothelial cushions of the latter. It would thus establish a complete par- tition between the two auricles were it not for the fact that secondary perforations arise in it before its free edge meets the endothelial cushions (Fig. 203), logical significance as the fora- men ovale in the mammalian heart, and persist through the period of incubation, closing soon after hatching. (d) The Cushion-septum (Sep- tum of the Auricular Canal). This septum completes the en- tire system by uniting together the three septa already consid- ered. It forms as two cushion- like thickenings of the endothe- lium in the floor and roof re- spectively of the auricular canal (cf. Figs. 202, 203 and 204). These cushions rapidly thicken so as to restrict the center of the atrioventricular aperture, and finally, fusing together, di- vide the latter into two verti- cally-elongated apertures, right and left respectively. The time of formation of this large endo- cardial cushion dividing the au- ricular canal is coincident with the formation of the other septa. (e) Completion of the Septa. These have the same physio-. FiG. 203. — Reconstruction of the heart of a chick embryo of mm. head-length, seen from right side. Part of the wall of the right auricle is cut away. (After Masius.) B. Co., Bulbus cordis. D. C, Duct of Cuvier. E. C. d., v., Dorsal and ventral endothehal cushions. Opening of the sinus venosus into the right auricle. , , Primary and secondary ostia or inter-auricular con- nections. Thus bv the end of the fifth or the beginning of the sixth day of incubation, the heart is prepared for the rapid completion of a double circulation. The embryonic circulation is never completely double, however, for the reason that the embryonic respiratory organ (allantois) belongs to the aortic system, and fu
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