Figure 19.

Stages 39 and 40: Anterior–posterior axis reads left to right in both images. Scale bar = 1 mm. Stage 39 (9 days). Stage 40 (9 ½ days). Anterior–posterior axis reads left to right. Abbreviations: al, alula; cl, cloaca; el, eyelid; er, opening to ear; et, egg tooth; fg, feather germ; fl, forelimb; hal, hallux; hl, hindlimb; ica, internal carotid artery; le, lens vesicle; md, mandibular process; ms, mesencephalon; mx, maxillary process; na, nail; nar, nares; pr, pigmented retina of eye; tbt, tibiotarsus; tl, tail; tmt, tarsometatarsus; v1, lateral ventricle of telencephalon. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]