STAGE 21. Age 22 ½ h.; length 1.9-2.0 mm.

Ext. Crit.: *Suture of neural tube completely closed*. *Delimitation of frontal field by pigment lines*. Primary eye vesicles showing through in the form of two separate, obliquely placed oval spots; *beginning of protrusion of eyes*. Lateral outline of embryo just becoming concave, ventral outline flat.

Int. Crit.: Middorsal suture of neural tube disappeared. First indication of development of cephalic flexure and of rhombencephalic roof formation. Closure of canalis neurentericus. Primary eye vesicles formed. Ear placode visible for the first time as thickening of sensorial layer of ectoderm.
*Segregation of maxillary and mandibular portions of mesectoderm*. *Anterior 8-9 somites segregated*. First indication of pronephros anlage.