It is formed by the convergence of some mesodermal cells at the head end of the oropharyngeal membrane at the anterior edge of the blastoderm.

pericardial cavity

heart tube

Cardiac area (rostral side of oropharyngeal membrane) → pericardial cavity

Cardiac cord (mesenchymal cells on the ventral side of the pericardial cavity) → heart tube (pair)

Partial fusion of the heart tube (head and tail are relatively fixed)

Complete fusion of heart tubes

Each segment of the heart tube grows at different rates

truncus arteriosus

sinus venosus

The heart tube bends to form a U-shaped spherical chamber

The cardiac bulb and ventricle curve to the right, ventrally, and caudally

The atria and sinuses curve to the left, dorsally, and cranially

The ball room trip becomes S-shaped

The heart begins to take shape

Atrioventricular canal → (thickening of endocardial tissue in dorsal and ventral centers) → dorsal, ventral endocardial cushions

First room partition

First room hole

The second room hole (the first room hole is closed)

Second room partition

foramen ovale

interventricular aperture

ventricular septal muscle

Ventricular septum (weekend of seventh week)

aortopulmonary septum

semilunar valve

cardiovascular

cardiovascular wall

Blood island → extraembryonic primitive vascular network

intraembryonic primitive vascular network

primitive vascular access

Mesenchyme around the cleft (day 18-20) → capillaries → intraembryonic capillary network Extraembryonic capillary network → primitive cardiovascular system (week 3) → establishment of blood circulation

Composition (pairs)