<Tr> <Th_colspan="2"> Identifiers </Th> </Tr> <Tr> <Td_colspan="2"> Anatomical terminology (edit on Wikidata) </Td> </Tr> <P> Heart development refers to the prenatal development of the human heart . This begins with the formation of two endocardial tubes which merge to form the tubular heart, also called the primitive heart tube, that loops and septates into the four chambers and paired arterial trunks that form the adult heart . The heart is the first functional organ in vertebrate embryos, and in the human, beats spontaneously by week 4 of development . </P> <P> The tubular heart quickly differentiates into the truncus arteriosus, bulbus cordis, primitive ventricle, primitive atrium, and the sinus venosus . The truncus arteriosus splits into the ascending aorta and pulmonary artery . The bulbus cordis forms part of the ventricles . The sinus venosus connects to the fetal circulation . </P>

When does the heart form in a human fetus