Sea urchin gastrulation.
Invagination of the vegetal plate in the single-layered
blastula forms the blastopore, which will become the anus.
Further differentiation yields a gastrula with three embryonic tissue layers.
Infolding forms the archenteron,
which later develops into the digestive tube
with a second opening that becomes a mouth.
Mesenchyme cells migrate into the blastocoel,
and will later form the skeleton.