The heart develops from two symmetric primordia, which then merge into a single tube located in the neck. Due to the rapid growth of the tube in length, it forms an S-shaped loop). The first contractions of the heart begin at a very early stage of development, when the muscle tissue is barely distinguishable. The S-shaped heart loop distinguishes the anterior arterial, or ventricular, part, which continues into the truncus arteriosus, which is divided into two primary aorta, and the posterior venous, or atrial, into which the yolk-mesenteric veins flow, vv. omphalomesentericae.
In this stage, the heart is single-cavity, its division into the right and left halves begins with the formation of the atrial septum. By growing from top to bottom, the septum divides the primary atrium into two – the left and right, and in such a way that subsequently the confluence of the hollow veins are in the right, and the pulmonary veins – in the left.
The septum of the atria has a hole in the middle, foramen ovale, through which the fetal part of the blood from the right atrium enters directly into the left. The ventricle is also divided into two halves by a septum, which grows from the bottom towards the atrial septum, without completing, however, the complete separation of the ventricular cavities.
Sulci, sulci interventriculares appear on the outside according to the boundaries of the ventricular septum. Completion of the septum formation occurs after the truncus arteriosus, in turn, is divided by the frontal septum into two trunks: the aorta and the pulmonary trunk. The partition dividing the truncus arteriosus into two trunks, extending into the ventricular cavity towards the ventricular septum described above and forming the pars membranacea septi interventriculare, completes the separation of the ventricular cavities from each other.
The right atrium is adjacent to the original sinus venosus, which consists of three pairs of veins: the common cardinal vein, or the Cuvier duct (brings blood from the entire body of the embryo), the yolk vein (brings blood from the yolk sac) and the umbilical vein (from the placenta). During the 5th week, the hole leading from the sinus venosus to the atrium expands greatly, so that eventually the wall becomes the wall of the atrium itself. The left process of the sinus, together with the left duct duct that flows here, remains and remains as sinus coronarius cordis.
When flowing into the right atrium, sinus venosus has two venous valves, valvulae venosae dextra et sinistra. The left valve disappears, and valvula venae cavae inferioris and valvula sinus coronarii develop from the right valve. As a developmental abnormality, the 3rd atrium can be obtained, representing either the stretched coronary sinus into which all the pulmonary veins fall, or a separated part of the right atrium.