Summary of Events:
To recapitulate the sequence of events (Table 2.04): Angiogenic tissue and the vitelline veins appear by 18 days after ovulation. This is prior to the beginning of the linear division of the body into somites. By 19-20 days, the 1-2 somite stage, the endothelial heart tube and the umbilical veins have appeared. By 21 days (4 somites), the heart tube has fused. One day later, at the 6 somite stage, the heart tube has differentiated into atria, ventricle, and bulbus.
At 25 days of age when 14-16 somites are present, the primitive atria fuse, and the bulboventricular loop is formed. The pharyngeal arch arteries (gill arches) appear at 26-27 days of age. The embryo now has 20 somites and is 10 mm long.
The anterior cardinal vein appears at 28 days of age (26 somites), and the posterior cardinal vein appears one day later. Septum primum, which begins to divide the common atrium, appears on day 32. The embryo is now 5 mm long.
The interventricular septum appears at 33 days of age (6 mm). The bulbar ridges fuse at day 43. By 46 days of age the interventricular foramen has closed, thus the right and left ventricles exist. One day later, with the embryo 18 mm in length, the septum secundum appears.
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