Day 1:
6 to 10 h - First kidney-like cells (pronephros) begin to form.
8 h -Appearance of primitive streak.
10 h -Yolk sac (embryonic membrane) begins. Functions include: a) blood formation; b) yolk digestion; c) yolk absorption; d) food provision after hatching. Mesoderm appears; embryo oriented at 90° angle to egg's long axis; mesonephros begins.
18 h -Primitive gut begins; primordial germ cells appear in germinal crescent.
20 h -Vertebral column begins.
21 h -Appearance of neural groove, nervous system.
22 h -Appearance of first pair of somites (block-like segments) and head.
23 to 24 h -Blood islands, vitelline (yolk sac) circulation, blood, heart,blood vessels begin (2 to 4 somites).
Day 2:
25 h -Appearance of eye; vertebral column visible; embryo begins to turn on left side (6 somites).
28 h -Ear begins (7 somites).