Figure 8.

Stages 17 and 18: Anterior–posterior axis reads left to right in both images. Scale bar = 1 mm. Stage 17 (3 ¼ days). Stage 18 (3 ⅓ days). Abbreviations: af, amniotic fold; all, allantois; at, atrium of heart; bc, bulbus cordis; cf, choroid fissure; di, dienchepalon; ep, epiphysis; flb, forelimb bud; hlb, hindlimb bud; lbf, lateral body of foregut; le, lens vesicle; ms, mesencephalon; mt, metencephalon; my, myelencephalon; opc, optic cup; ot, otic vesicle; pa, pharyngeal arch; rp, Rathke’s pouch; sm, somite mesoderm; tb, tailbud; te, telencephalon; tr, truncus arteriosus; vt, vitelline artery. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]