part of blastopore BTO:0001695 BrendaTissueOBO During gastrulation cells on the surface of the embryo move into the interior to form the mesoderm and endoderm. The opening formed by this invagination of cells is the blastopore. It is an opening from the archenteron, the primitive gut, to the exterior. In some animals this opening becomes the anus, whilst in others it closes up and the anus opens at the same spot or nearby. archenteron