STAGE 16. Age 18 ¼ h.; length 1.5 -1.6 mm.

Ext. Crit.: *Mid neural fold stage*. *Anterior part of neural plate rectangular*; darkly pigmented eye anlagen present; *neural plate sharply constricted in the middle*. Inner ridges on neural folds forming angle of about 90° with neural plate in rhombencephalic region.

Int. Crit.: First visible participation of epithelial layer of ectoderm in neural plate. *Distinct elevation of neural folds along entire length of neural anlage*. *Eye anlagen forming deep depressions in antero-lateral edges*. Left and right heart anlagen begining to fuse.