STAGE 27. Age 1 d., 7 ¼ h.; length 3.4-3.7 mm.

Ext. Crit.: Lateral flattening of eyes. *Fin translucent*, except for region just behind anus. *Tail bud formation accentuated in lateral outline*.

Int. Crit.: First part of n. ophthalmicus profundus distinct. *Retinal layer of optic vesicle invaginating at antero-dorsal margin*; beginning of lens formation as thickening of sensorial layer of ectoderm. *Ear vesicle closed, but still connected with ectoderm*.
*19 Somites segregated*. Endocardial anlage delimited. *Wolffian duct extending to level of trunk somite 5*.
*First contact of ectoderm and entoderm in 3^rd visceral pouch (formation of 1^st branchial arch)*.