On day 19 of development,
a pair of vascular elements, the"Endocardial tubes", form.
After about two weeks, these expansions, the membranous"septum primum" and the muscular"Endocardial cushions", fuse to form the four chambers of the heart.
After about two weeks, these expansions, the membranous"septum primum" and the muscular"Endocardial cushions", fuse to form the four heart chambers of the heart.
On day 28, areas of tissue in the heart tube begin to expand inwards; after about two weeks, these expansions,
the membranous"septum primum" and the muscular"Endocardial cushions", fuse to form the four chambers of the heart.