Germ band extension in Drosophila positions posterior segments initially:
Germ band extension involves rapid elongation of embryo along anteroposterior axis after mesoderm invagination, driven by convergent extension via cell intercalation, oriented cell division and changes in cell shape controlled by pair-rule genes and planar cell polarity. As posterior midgut moves dorsally, germ band lengthens and its caudal end originally near posterior pole displaces anteriorly to position behind head rudiment on dorsal side. This movement brings future abdominal segments temporarily to dorsal-anterior region. Subsequent germ band retraction reverses movement restoring poster
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 10: Germ band extension movements.
