A L. variegatus early gastrula, viewed from the side (courtesy of Dr. Chuck Ettensohn, Carnegie-Mellon Univ).
The vegetal plate undergoes invagination to produce the archenteron. Since sea urchins are deuterstomes, the blastopore forms the anus of the larva later in development. The vegetal plate is an epithelial monolayer, and so the process of primary invagination seems like a deceptively simple case of invagination. However, appearances can be deceiving! The mechanisms of primary invagination are not well understood. However, several recent experiments indicate at least some of the requirements for primary invagination to occur normally. The next several pages explore possible mechanisms of primary invagination.