Figure 5: Development of the lung in equine embryos and fetuses. (A-B) Note the histological features of the lung bud in embryos with 21 days of gestation. Note the beginning of the formation of the primary bronchi (PB) within the parenchyma formed by mesenchyme (M). The bronchi have stratified epithelium (E) limiting their lumen (L). Blood cells (C) are observed both inside the bronchi and in the mesenchyme. (C-F) Lung development during days 24-49 of development. (D-F) During this period, the lung undergoes divisions of the branches of the secondary bronchi (SB), with lobes forming on its surface that are observed by both histology and scanning electron microscopy. (G) Note in the lung of 40-day-old embryos the presence of the somites (S), the primitive lung (Lu), and its syntopy with the liver (L), primary bronchi (PB), secondary bronchi (SB), and lung parenchyma (LP).