~21st day of embryonic development
||Human Homo sapiens
||Männer J, Wessel A, Yelbuz TM. How does the tubular embryonic heart work? Looking for the physical mechanism generating unidirectional blood flow in the valveless embryonic heart tube. Dev Dyn. 2010 Apr239(4):1035-46. doi: 10.1002/dvdy.22265. abstract & p.1035 left columnPubMed ID20235196
||Britten S, Soenksen DM, Bustillo M, Coulam CB. 1994. Very early (24–56 days from the last menstrual period) embryonic heart rate in normal pregnancies. Hum Reprod 9: 2424–2426. AND Wisser J, Dirschedl P. 1994. Embryonic heart rate in dated human embryos. Early Hum Dev 37: 107–115.PubMed ID7714168, 8088227
||Abstract: "The heart is the first organ to function in vertebrate embryos. The human heart, for example, starts beating around the 21st embryonic day." P.1035 left column: "Introduction: The heart is the first organ to function in vertebrate embryos. The human heart, for example, starts its contractile activity around the 21st day of embryonic development (primary sources). At this developmental stage, embryonic vertebrate hearts are seen as relatively simple tubular blood vessels built up by an inner endocardial tube in contact with the blood, a middle layer of a cell-free extracellular matrix called the cardiac jelly (Davis, 1924), and an outer myocardial tube in direct contact with the pericardial fluid (Fig. 1)." See Suemori 2006 PMID 16879558 p.67 right column 2nd paragraph: "ES [embryonic stem]-cell aggregates
will form simple EB [embryoid bodies] in a few days. They can be cultured in suspension until apparent cell differentiation is observed. In 2 to 3 weeks, beating heart muscle and hematopoietic cells may be observed. Alternatively, EB can be plated from the suspension culture to a tissue-culture dish at any time. EB will attach to the dish and undergo cell
differentiation into various tissues such as neurons and cardiac muscles."