The sponge, a many celled animal, begins its life as a single-cell, the egg. After division of cells and formation of blastula, the young larva leaves the parent. The next transformation consists of a pushing in of one side. This depression deepens and gradually results in the formation of a two layered sac know as the gastrula.
Winchell, Alexander Sketches of Creation (New York, NY: Harper & Brothers, 1870)