Primitive Cardiac Tube Development

As the primitive heart continues to develop, it begins to lengthen. From day 22 to day 24, the heart tube doubles in length. The ends of the tube are fixed, therefore as it lengthens, it is forced to bulge and twist within the pericardial sac. It is proposed that differing rates of growth along the tube are responsible for looping of the heart.

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The heart doubles in length between days 22 and 24.

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At about the same time that the primitive heart begins to pulsate, cells of the _____ and _____ are beginning to appear.