part of part of immediately preceded by immediately preceded by occurrent embryogenesis phase blastula period gastrula period The period that immediately follows the blastula period and precedes organogenesis. It is characterized by extensive movements of cells in the vegetal region of the embryo. This period culminates in the formation of a multilayered embryo composed of three primary germ layers (ectoderm, mesoderm, and endoderm). The cell movements of gastrulation include ingression of migratory, mesenchymal cells into the blastocoel and invagination of the vegetal plate that creates the primitive gut (archenteron). UBERON:0000109 ecao_developmental_stage