Changes

Two major blood vessels which form ventral to the neural tube become the left and right dorsal aortae. In the mesenchyme adjacent to the truncus arteriosus, another series of paired aortic arch arteries develop that join the dilated end of the truncus arteriosus with the dorsal aortae. Branches of the dorsal aortae, the intersegmental arteries, supply the developing somites. Additional branches supply the yolk sac through the vitelline arteries and the umbilical arteries supply the allantois. On each side of the developing embryo, the cranial and caudal cardinal veins fuse forming the common cardinal veins which enter the sinus venosus. At this stage of morphogenesis, the developing mammalian cardiovascular system and the fully formed circulatory system of the fish are very similar to each other.

Two major blood vessels which form ventral to the neural tube become the left and right dorsal aortae. In the mesenchyme adjacent to the truncus arteriosus, another series of paired aortic arch arteries develop that join the dilated end of the truncus arteriosus with the dorsal aortae. Branches of the dorsal aortae, the intersegmental arteries, supply the developing somites. Additional branches supply the yolk sac through the vitelline arteries and the umbilical arteries supply the allantois. On each side of the developing embryo, the cranial and caudal cardinal veins fuse forming the common cardinal veins which enter the sinus venosus. At this stage of morphogenesis, the developing mammalian cardiovascular system and the fully formed circulatory system of the fish are very similar to each other.

===Septation===

===Atrial Septation===

Septation of the tubular heart begins in the atrioventricular canal. Two masses of cardiac mesenchymal tissue, known as endocardial cushions, extend towards each other and fuse. The fused endocardial cushions form the septum intermedium, which divides the common atrioventricular canal into left and right atrioventricular canals.

Septation of the tubular heart begins in the atrioventricular canal. Two masses of cardiac mesenchymal tissue, known as endocardial cushions, extend towards each other and fuse. The fused endocardial cushions form the septum intermedium, which divides the common atrioventricular canal into left and right atrioventricular canals.

During proliferation of the endocardial cushions the initial atrial septum forms inside the common atrial chamber as a crescent-shaped ridge called the septum primum. The septum primum emerges from the dorsal wall of the common foetal atrium and extends towards the endocardial cushions. The septum primum becomes progressively smaller as the septum primum advances and disappears altogether when the septum primum fuses with the endocardial cushions. Before closure of the foramen primum, however, apoptosis in the central part of the septum primum results in the formation of a new foramen between the left and right atria, the foramen secundum. A second membrane, the septum secundum, arises from the dorsal wall of the right atrium and extends towards the septum intermedium. The opening which persists between the free edge of the septum secundum and the foramen secundum is known as the foramen ovale.

During proliferation of the endocardial cushions the initial atrial septum forms inside the common atrial chamber as a crescent-shaped ridge called the septum primum. The septum primum emerges from the dorsal wall of the common foetal atrium and extends towards the endocardial cushions. The septum primum becomes progressively smaller as the septum primum advances and disappears altogether when the septum primum fuses with the endocardial cushions. Before closure of the foramen primum, however, apoptosis in the central part of the septum primum results in the formation of a new foramen between the left and right atria, the foramen secundum. A second membrane, the septum secundum, arises from the dorsal wall of the right atrium and extends towards the septum intermedium. The opening which persists between the free edge of the septum secundum and the foramen secundum is known as the foramen ovale.