During which stage of early embryonic development do the primary germ layers form?

Indicate which primary tissue classes derive from each embryonic germ layer.

Most systems are operational in the fetus by four to six months. Which system is the exception to this generalization, affecting premature infants?

(a) the circulatory system

(b) the respiratory system

(c) the urinary system

(d) the digestive system

In the fetus, the ductus arteriosus carries blood from

(a) the pulmonary artery to the pulmonary vein

(b) the liver to the inferior vena cava

(c) the right ventricle to the left ventricle

(d) the pulmonary trunk to the aorta