During the third week of fetal life, the mesoderm of the septum transversum at the cranial end of the embryo reorganises to give a distinct cavity, the pericardial coelom. The lining of this cavity is primitive fibrous pericardium.
Mesenchymal cells aggregate in the ventral part of the cavity to form the the primitive heart tube. The tube is attached dorsally to the wall of the cavity by a double investing layer of dorsal mesocardium. The latter is the progenitor of the serous mesocardium.
The dorsal mesocardium applies itself to the surfaces of the heart and the roots of the great vessels as they from and reposition during development. Eventually, the dorsal mesocardium degenerates from its dorsal attachments apart from the points at which it sheaths the passage of the great vessels to and from the heart and some connections between these regions. The area of dorsal mesentery loss from between the veins - pulmonary and vena cavae - and arteries - aorta and pulmonary artery - is termed the transverse sinus. The oblique sinus is formed from dorsal mesentery loss between the pulmonary veins.
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