Specification of germ layers in amphibians begins:
Axis and germ layer specifications in amphibians rely on maternally deposited RNAs and proteins localized during oogenesis rather than zygotic transcription initially. VegT, Vg1, Wnt11 mRNAs accumulate vegetally months before fertilization through localization machinery involving cytoskeleton. These determinants pre-pattern embryo, ensuring vegetal cells are biased toward endoderm even before sperm entry occurs. Post-fertilization events like cortical rotation merely redistribute dorsalizing factors dorsally, but initial animal-vegetal polarity exists prior to fertilization. This explains why isolated vegetal explants differentiate into gut without any additional external signals or zygotic input.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 2: Maternal mRNA localization before fertilization specifies germ layers.