In the embryo, the respiratory system originates as a ventral outgrowth from the floor of the primitive pharynx, the anterior part of the foregut. The outgrowth then extends inferiorly and divides into right and left bronchial buds, each of which undergoes repeated dichotomous branchings. The primary outgrowth becomes the trachea, each bronchial bud a main bronchus, and the orders of branchings the smaller bronchi, bronchioles, and terminal alveoli. Thus, the entire system has a lining which is of endodermal origin, being derived from the lining of the foregut. Respiratory tissue initially has a gland-like appearance of epithelium (endodermis)-lined alveoli embedded in mesoderm. The mesoderm forms the accessory coats of the system, e.g., connective tissue, muscle.
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