FIG 3.14 Image in four-chamber plane from a human embryo at Carnegie stage 16 The section shows how the primary atrial septum, with its mesenchymal cap, is growing toward the superior atrioventricular (AV) cushion The cranial origin of the septum has broken down to form the secondary foramen The primary atrial foramen (bracket) is the space between the mesenchymal cap and the atrioventricular cushion Note the venous valves marking the boundary between the right atrial cavity and the systemic venous sinus FIG 3.15 Drawing based on the illustration of Wilhelm His the Elder showing how the vestibular spine grows into the space between the leading edge of the primary septum and the atrioventricular canal The mesenchymal tissue of the spine, together with the mesenchymal cap on the primary septum, then muscularize to form the buttress at the base of the atrial septum, thus anchoring the septum firmly against the fused atrioventricular cushions (Fig 3.16) Subsequent to these changes, with the atrial septum separating the newly formed right and left atriums, a secondary foramen remains present at the site of its breakdown at the atrial roof This hole is an essential part of the fetal circulation, permitting the richly oxygenated placental return to reach the left side of the developing heart, and then the developing brain through the aorta As the pulmonary veins are incorporated into the atrial roof, the ensuing folding between the pulmonary and systemic venous structures creates a cranial buttress against which the flap of the primary septum is able to close (see Fig 3.16) The space between the flap valve and the fold is the definitive oval foramen The foramen remains patent during fetal life, closing postnatally in most but not all individuals.9 FIG 3.16 Image prepared from an episcopic dataset from a mouse at embryonic day 17.5, which is just prior to birth The double-headed arrow shows the oval fossa, with the primary atrial septum forming its floor The cranial margin of the fossa is the infolding between the attachments of the systemic veins to the right atrium, and the pulmonary vein to the left atrium The inferior margin of the fossa is formed by the buttress derived by muscularization of the mesenchymal cap and vestibular spine More cranially still, the flap valve formed from the primary septum has yet to fuse with the superior fold This does not happen until after birth The space between the left venous valve and the septum is the small part of the body of the developing atrium remaining on the right side SCV, Superior caval vein The primary septum itself initially grows between the site of the left venous valve and the orifice of the newly formed pulmonary vein All the tissue to the left of the venous valve is mediastinal myocardium This tissue forms the body of the developing atrium The larger part of this body is committed to the definitive left atrium, with only a small part remaining in the right atrium between the left venous valve and the septum (see Fig 3.16) Each atrium eventually possesses a part of the body derived from mediastinal myocardium, with the larger part committed to the morphologically left atrium Each atrium also possesses an appendage, formed by bilateral ballooning from the initial lumen of the primary heart tube The chambers also possess a vestibule, derived on each side from the initial primary myocardium of the atrioventricular canal