In it cleavage gives rise to a solid mass, which divides by delamination into two layers, the ectoderm and endoderm.
It arises, however, by delamination from the ectoderm, the tube being a secondary acquisition.
The two layers are as before the epiblast and hypoblast, and the process by which they originate is known as delamination.
delamination is constant (with the doubtful exception of some Tubularid) amongst the Hydromedus and Siphonophora.
delamination de·lam·i·na·tion (dē-lām'ə-nā'shən)
A splitting or separation into layers.
The splitting of the blastoderm into two layers of cells to form a gastrula.