The tubular heart (Cor tubulare, S10)

At S10, the endocardial plexus becomes a longitudinally orientated tube called the endocardial tube, or the tubular heart. The epithelium of the pericardial cavity neighbouring the endocardium thickens to form the myocardium or myocardial mantle. A cell-free space filled with heart jelly, secreted by the myocardial cells, is formed between the endocardial tube and the myocardium. The pericardial cavity becomes invaginated by the endocardial tube. The visceral layer and the parietal layer come into close contact through this invagination. The visceral layer (myocardium) ensheathes the endocardial tube, forms a dorsal mesocardium and continues into the parietal layer (pericardium). Cells then grow from the serous epithelium of the sinus venosus to cover the myocardium and form the epicardium. Due to this, the pericardial cavity becomes covered with epicardium, which lies on top of the myocardium, and the pericardium, which defines the perimeter of the cavity.