|The rostral extremity of the neural tube closes in embryos
with usually about 15-18 somite pairs and defines this stage. The
otic pit becomes progressively more indented but not closed, the mandibular
process of the 1st branchial arch is clearly visible. The 3rd branchial
arch becomes visible late in the stage. An increasingly prominent
ridge on the lateral body wall, approximately at the level of the
8th-12th somite, indicates the site of the future forelimb bud.
Absent: forelimb bud.
All scale bars in microns.