STAGE 23. Age 1 d., ¾ h.; length 2.2-2.4 mm.

Ext. Crit.: *Jaw- and gill-areas completely separated by a groove*. Ventral outline of embryo more concave.

Int. Crit.: *Segregation of prosencephalon into tel- and diencephalon*. Spinal cord completeed at anterior end. First appearance of olfactory placodes as thickenings of sensorial layer of ectoderm. *First slight depression in ear placode underneath epithelial layer of ectoderm*.
*Segregation of 1^st branchial portion of mesectoderm*. *Approximately 12 somites segregated*. Beginning of segregation of nephrotomes; Wolffian duct anlage indicated.
*First contact of ectoderm and endoderm in 1^st visceral pouch (formation of mandibular arch); 3^rd visceral pouch indicated*.