Development of Heart in frog

Heart is a derivative of mesoderm and it begins to develop al about the tail bud stage. The heart develops under the inductive influence of the archenteric floor. The splanchnic mesenchymal cells grow ventrally toward the mid - line below the pharynx and contribute to the formation  of heart. The splanchnic mesoderm at the region of pharynx proliferates and the cells aggregate to form two small masses called endocardial cells. They become organised into two tubes known as endocardial tubes which fuse together to form a single tube, the rudiment of heart. Bilateral symmetry is evident in the development of heart during early stages.

Soon the mesoderm converges on either side of the endocardial tube and encloses them. The growing ends of mesoderm meet at first ventrally and fuse below the endocardium. There appears a ventral mesocardium temporarily. Soon it disappears. Dorsally, the mesodermal sheets meet just above the endocardium and below the pharynx. When the fusion of the two mesoderms is complete, the heart is suspended from above by a dorsal mesocardium. The endocardium is now surrounded by two mesodermal layers - the inner myocardium and the outer pericardium. The space in between myocardium and pericardium is called pericardial cavity. The myocardium becomes highly muscular and gives rise to the striated, synsytial and involuntary muscles of the heart. At the level of heart, the mesocardia disappear both dorsally and ventrally. This is accomplished in part by the rapid growth and expansion of heart itself. The rudimentary heart consists of a simple tube which becomes divided into three chambers, two thin walled auricles and one thick - walled ventricle. Posteriorly, it receives two vitelline veins and anteriorly it loads into truncus arteriosus or ventral aorta. 

Flexion and torsion of the heart tube 

 

The development of heart is completed within a limited space provided by the surrounding body tissues and organs. The developing heart tube undergoes flexion and torsion within the pericardium so that the heart becomes a reversed S-shaped coiled tube . The posterior part of the original tube becomes folded dorsally and anteriorly Then, anterior part of the heart is folded ventrally and posteriorly. Thu there is a reversal in original antero - posterior relation of heart with respej to embryo. The ventricular part of heart is brought downward and auricular part upward . The terminal parts of heart, which are held in position by the dorsal mesocardium are the posterior sinus venosull, and anterior truncus arteriosus. The point of fusion of the vitelline veins to the heart is called sinus venosus which represents the posterior limit. Soon there develops a longitudinal sheet of endocardium from the roof of the atrial chamber and that divides auricle into left and right chambers. The ventricle remains undivided. The truncus arteriosus becomes partially subdivided by vertical and longitudinal septum and gives rise to the ventral paired aortae which are connected with various aortic arches.