29.The answer is a.(Sadler, pp 157–162. Moore and Persaud, Developing,
pp 60–69, 80–81, 336–338.)The fusion of the dorsal aortae occurs through
lateral folding. Fusion of the endocardial heart tube and incorporation of
the yolk sac into the primitive gut also occurs as a result of lateral folding.
Craniocaudal folding (answer b)establishes the definitive head and tail
regions of the embryo. Fusion is already complete at the time that looping
of the heart tube occurs (answer c).Gastrulation(answer d)establishes
the three germ layers (trilaminar disk), and neurulation establishes the
neural groove with two neural folds. Neurulation is the formation of the
neural tube (answer e).
Embryology: Early and General Answers 95