Progression of ACE synthesis from stage HH4 to HH18. Whole-mount in situ hybridization with ACE digoxigenin (Dig)–labeled riboprobe (dorsal view) at stage HH4 (A), HH6 (B), HH9 (C), and HH18 (D). (A) ACE expression is initially limited to a horseshoe-shaped pattern in the anterior extraembryonic area. (B) The expression spreads laterally and caudally on both sides of the embryo. (C) Finally, ACE covers the extraembryonic area. (D) Ventral view of the embryo on day 3 showing ACE labeling in the entire area vasculosa.