STAGE 12. Age 13 ¼ h.; length 1.4 -1.5 mm.

Ext. Crit.: *Medium yolk plug stage*. Yolk plug circular; *diameter somewhat less than ¼ of diameter of egg (+/- 25°)*. More and less pigmented fields radiating from yolk plug.

Int. Crit.: Neural anlage extended craniad; *neural and epidermal areas clearly distinguishable anteriorly*.

*Mesodermal mantle reaching some distance from the animal pole at dorsal side*; beginning of segregation of prechordal and chordal areas.

*Archenteron extending over more than 90°*; first indications of widening.