part of part of immediately preceded by immediately preceded by occurrent late mesenchyme blastula episode blastopore formation stage blastopore gastrula stage The embryo is characterized morphologically by a slight invagination of the vegetal plate. This invagination has generated the anterior portion of the archenteron (primitive digestive tract) and the blastopore, an opening that will persist as the larval anus. blasto-G ecao_developmental_stage invagination episode