The developing bronchial arteries in a fetus of the twelfth week

In the human fetus bronchial arteries appear, as such, between the ninth and twelfth week as highly variable arteriolar outgrowths from the aorta itself or from proximal portions of right (or left) posterior intercostal rami of the aorta. Thus, they constitute a secondary system implanted relatively late upon the peripheral coats of the bronchial tree and large pulmonary vessels. The process is thought to be an inductive one depending partly upon the approximation of parts during the descent of heart and lungs into the thorax and partly upon the advancing degree of maturity of the bronchial tree. In the fetus of the twelfth week, to be described below, there are three bronchial arteries to the left lung and at least one to the right. The lowest of these (the left aorticobronchial artery) can be traced along the superficial coasts of the left lower lobe bronchus to the level just preceding that in which cartilages are differentiating: for example, to the third of what will comprise, eventually, some 25 generations of branches of the posterior basal segmental bronchus. Beyond this shifting level of differentiation the segmental pulmonary arteries constitute the sole arterial supply to the bronchi and parenchyma of the lung. It is assumed from recent reports in the literature that the bronchial arterioles will have established continuity with pulmonary arteries and with the pulmouary parenchyma not later than the twenty‐sixth week of gestation.