Changes

===Atrial Septation===

===Atrial Septation===

Septation of the tubular heart begins in the atrioventricular canal. Two masses of cardiac mesenchymal tissue, known as endocardial cushions, extend towards each other and fuse. The fused endocardial cushions form the septum intermedium, which divides the common atrioventricular canal into left and right atrioventricular canals.

Septation of the tubular heart begins in the atrioventricular canal. Two masses of cardiac mesenchymal tissue, known as endocardial cushions, extend towards each other and fuse. The fused endocardial cushions form the Septum Intermedium, which divides the common atrioventricular canal into left and right atrioventricular canals. The septum is shifted to the right allowing the left side of the heart to become bigger than the right

During proliferation of the endocardial cushions the initial atrial septum forms inside the common atrial chamber as a crescent-shaped ridge called the septum primum. The septum primum emerges from the dorsal wall of the common foetal atrium and extends towards the endocardial cushions. The septum primum becomes progressively smaller as the septum primum advances and disappears altogether when the septum primum fuses with the endocardial cushions. Before closure of the foramen primum, however, apoptosis in the central part of the septum primum results in the formation of a new foramen between the left and right atria, the foramen secundum. A second membrane, the septum secundum, arises from the dorsal wall of the right atrium and extends towards the septum intermedium. The opening which persists between the free edge of the septum secundum and the foramen secundum is known as the foramen ovale.

As the endocardial cushions proliferate the initial atrial septum forms in the common atrial chamber. The Septum Primum emerges from the roof of the common atrium and extends down through the chamber, eventaully fusing with the Septum Intermedium to create a seperate left and right atria. Programmed cell death occurs and a large hole is created in the septum primun allowing blood to flow between the left and right. A muscular sheet known as the Septum Secundum then grows on the right of the Septum Primum. This occludes the majority of the hole in the Septum Primum, only the small Foramen Ovale is left. The window in the Septum Primum is slightly offset from the window in the septum Secundum and this creates a valve like effect so that blood can only flow in one direction between the atriae.

===Ventricular Septation===

===Ventricular Septation===