The primary germ layer derived from vegetal hemisphere cells is:
Maternal mRNAs and proteins localized vegetally during oogenesis bias vegetal hemisphere toward endodermal fate, establishing first germ layer map. VegT, Vg1 localized there activate transcription factors Sox17, Mixer, GATA factors after fertilization, driving gut differentiation. Cells inheriting high VegT and Nodal signaling differentiate as endoderm forming archenteron lining, while equatorial cells receiving intermediate Nodal become mesoderm and animal cap lacking VegT defaults to ectoderm. Lineage tracing confirms large yolky vegetal blastomeres predominantly contribute to archenteron lining, while mesoderm, ectoderm, neural tissues originate more animally, reflecting conserved anamniote pattern.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 8: Vegetal hemisphere endoderm origin maternal determinants.