Cnidaria begin to differentiate their nervous systems in the late gastrula. In due course, the blastula changes into a more differentiated structure called the gastrula. This suggested that the gastrulation and the gastrula stage are universal for eumetazoans. Named by William Ballard, the pharyngula stage follows the blastula, gastrula and neurula stages. In the early gastrula, most of the tissue in the embryo is not determined. Enterocoelous development begins once the embryo reaches the gastrula phase of development. The embryo during this process is called a gastrula. The neurula stage is preceded by the gastrula stage; consequentially, neurulation is preceded by gastrulation. Chicken turtle embryos go through a period of diapause in the late gastrula stage. By Day 1 the embryo has hatched as a ciliated gastrula stage (photograph). They become expressed in the anterior region of the dorsal ectoderm in late gastrula embryos. At this point gastrulation is complete, and the gastrula has a functional digestive tube. In triploblastic organisms the gastrula is trilaminar ("three-layered"). The gastrula then evolves to a dipleurula larva form, which is specific for the ambulacraria. In diploblastic organisms, such as Cnidaria and Ctenophora, the gastrula has only ectoderm and endoderm.