FIGURE 2.

Three‐dimensional models of human embryo specimens of stages 12 through 15. The external curvature of the body is largely determined by the morphology of the underlying neural tube. Up to four pharyngeal arches appear, with the third and fourth eventually beginning to recede from the surface. The upper and lower limb buds first appear at stages 12 (a) and 13 (b) respectively. Nasal discs first appear at stage 14 (c) and exhibit a central depression or nasal pit by stage 15 (d). The external opening of the coelomic cavity, which is still present at stage 12, is virtually already closed by stage 13. uplb: upper limb bud; lolb: lower limb bud; phar: pharyngeal arches (dashed lines indicate ongoing subdivisions); cp: cardiac prominence; fnp: frontonasal prominence; tb: tail bud; uv: umbilical vesicle; uc: umbilical cord; us: umbilical stalk; am: amniotic membrane (cut edge); cs: cervical sinus; mp: maxillary process; seg: segmented axial mesoderm; N: Nackengrube of His; nd: nasal disk; led: lens disk; lep: lens pit; me: mesencephalic prominence; npr: nasal process; np: nasal pit; e: eye