tubular heart tubular heart the tubular heart or primitive heart tube is the earliest stage of heart development. tubular heartdiagram to illustrate the simple tubular condition of the heart.heart of human embryo of about fourteen days.detailsdays21precursorsplanchnic mesodermgives rise toheartidentifierslatincor tubulareteheart_by_e5.11.1.1.1.0.5 e5.11.1.1.1.0.5 anatomical terminology from the inflow to the outflow, it consists of sinus venosus, primitive atrium, the primitive ventricle, the bulbus cordis, and truncus arteriosus. it forms primarily from splanchnic mesoderm. more specifically, they form from endocardial tubes, starting at day 21.