Changes

At this point blood enters the caridac tube at the posterior end. The bulbis cordis loops to the right and splits into two new bulges, the conotruncus and the right ventricle. This causes the primitive atrium to be displaced dorsally. The ventricles expand while the conotruncus dives into to further bulges, the truncus areteriosus (anterior) and the conus cordis (posterior). The atria expand dorsally and also on either side of the truncus arteriosis, this gives rise to the formation of two chambers, the left and right atria. The sinus venosus is located  on the dorsal aspect of the interconnected atria. The sinus venosus is composed of a left and right horn, the majority of the left horn is lost, meaning that the communication between the sinus venosus and the atria, the sinoatrial opening, is displaced to the right atria. This then means the left side of the atrium is devoid of blood vessels . The developing heart combats this by sprouting blood vessles that connect the left atrium to the lungs. The sprout splits into four channels, providing two vessels per lung.

At this point blood enters the caridac tube at the posterior end. The bulbis cordis loops to the right and splits into two new bulges, the conotruncus and the right ventricle. This causes the primitive atrium to be displaced dorsally. The ventricles expand while the conotruncus dives into to further bulges, the truncus areteriosus (anterior) and the conus cordis (posterior). The atria expand dorsally and also on either side of the truncus arteriosis, this gives rise to the formation of two chambers, the left and right atria. The sinus venosus is located  on the dorsal aspect of the interconnected atria. The sinus venosus is composed of a left and right horn, the majority of the left horn is lost, meaning that the communication between the sinus venosus and the atria, the sinoatrial opening, is displaced to the right atria. This then means the left side of the atrium is devoid of blood vessels . The developing heart combats this by sprouting blood vessles that connect the left atrium to the lungs. The sprout splits into four channels, providing two vessels per lung.

While this is taking place the major intraembryonic vessels are also being fromed from the dorsal mesenchyme.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.

While this is taking place the major intraembryonic vessels are also being fromed from the dorsal mesenchyme.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.