Changes

Within the cardiogenic plate, angiogenic cell clusters give rise to a horseshoe-shaped structure known as  the endocardial tube. The lateral limbs of the horseshoe-shaped construction form the left and right endocardial tubes. The wall of the tubular heart is composed of loosely organised myocardium that is several cells thick and is separated from the endocardium by a layer called the cardiac jelly. Many of the major intra-embryonic blood vessels including the dorsal aortae are formed at the same time as the endocardial tubes and extra-embryonic vessels. Mesodermal cells proliferate in a position cranial to the cardiogenic plate and form the septum transversium.

Within the cardiogenic plate, angiogenic cell clusters give rise to a horseshoe-shaped structure known as  the endocardial tube. The lateral limbs of the horseshoe-shaped construction form the left and right endocardial tubes. The wall of the tubular heart is composed of loosely organised myocardium that is several cells thick and is separated from the endocardium by a layer called the cardiac jelly. Many of the major intra-embryonic blood vessels including the dorsal aortae are formed at the same time as the endocardial tubes and extra-embryonic vessels. Mesodermal cells proliferate in a position cranial to the cardiogenic plate and form the septum transversium.

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.