From the gastrula type the higher sub- divisions of animal life probably diverged, as separate branches from a common trunk.
A gastrula is established by a mixed process of embole and epibole.
But how is an adult worm or vertebrate formed out of such a gastrula?
Both the blastosphere and gastrula often swim freely by flagella.
The formation of the gastrula and behaviour of the blastopore: together with the origin of the hypoblast.
This gastrula is the simplest form of the "person," and the two germinal layers are its sole organs.
This exceedingly important larval form, the "gastrula," makes its appearance in the ontogenesis of all tribes of animals.
The two layers of the gastrula may now be spoken of as ectoderm and entoderm.
At the same time they become clearer and permit a view of the interior of the gastrula.
The gastrula stage was the palingenetic repetition of the ancestral form of all Metazoa, the Gastræa.
gastrula gas·tru·la (gās'trə-lə)
n. pl. gas·tru·las or gas·tru·lae (-lē')
An embryo at the stage following the blastula, consisting of a hollow, two-layered sac of ectoderm and endoderm surrounding an archenteron that communicates with the exterior through the blastopore.
Plural gastrulas or gastrulae (gās'trə-lē')
An animal embryo at the stage following the blastula. The gastrula develops from the blastula by invagination (inpocketing), forming an inner cavity with an opening and causing the cells to be distributed into an outer layer (ectoderm) and an inner layer (endoderm). In complex animals such as vertebrates, a third layer (mesoderm) also forms. These layers later develop into the organs and tissues of the body. In vertebrates and other deuterostomes, the opening of the gastrula becomes the anus, while in protostomes (such as arthropods), it becomes the mouth. ◇ The development of an embryo from blastula to gastrula is called gastrulation. Compare blastula.