Schematics in left column depict quail embryos at the stage isolated and the red line denotes the plane of section. A–B: At E5, the outer epithelial basement membrane appeared dispersed (white arrowhead). Yellow arrowhead denotes the endodermal basement membrane. C–D: At E6, both the outer (white arrowhead) and endodermal (yellow arrowhead) basement membranes were unbroken. E–F: At E10, villi (V) were present and both basement membranes were continuous (arrowheads). G–H: At E16, the mesenchyme was condensed (compare F and H). The outer basement membrane was robust and unbroken (white arrowhead) while the mucosal basement membrane weakly stained with laminin (yellow arrowhead). Scale bars: 50µm (A, C, E, G,) and 10µm (B, D, F, H). En, endoderm; FL, forelimb; H, head; HL, hindlimb; L, lumen; Le, leg; M, mesenchyme; Mes, mesothelium; Mu, mucosa; OE, outer epithelium; V, villi; W, wing.