STAGE 13. Age 14 ¾ h.; length 1.5 -1.6 mm.

Ext. Crit.: *Slit-blastopore stage*. Neural plate faintly delimited. slight elevation of its rostral part and slight flattening of its caudal part. *Caudal part of median groove formed*.

Int. Crit.: Formation of neural anlage still restricted to sensorial layer of ectoderm; first more intimate attachment in dorsal midline between epithelial and sensorial layer of ectoderm, and between the latter and the underlying archenteron roof. Presumptive somite mesoderm double layered. *Archenteron extended to about 10° ventral to animal pole*; caudal archenteron ring-shaped around entoderm protuberance. Blastocoel rapidly decreasing in size.