The cordlike masses of splanchnic mesenchymal cells ventral to the primordial pericardial coelom that arrange themselves side by side to form the primordia of the endocardial heart tubes.
Comment:
[taxon-note] "day 18 [CS08] in human[https://embryology.med.unsw.edu.au/embryology/index.php/Intermediate_-_Primordial_Heart_Tube]"