Through a process called gastrulation, the cells of the blastula are reorganized to form the three primary germ layers of the gastrula that are present in all triploblastic organisms. The blastula is the resulting structure, consisting of at least 128 cells surrounding a cavity of mainly empty space, called the blastocoel. The image shows the defining differences between deuterostome and protostome embryonic development. Additionally, most of the deuterostomes display indeterminate cleavage, in which the developmental fate of each cell is not predetermined in the embryo and therefore each cell has the ability to develop into a complete embryo if isolated.
Radial cleavage is one of the defining features of the deuterostome development, contrasting the spiral cleavage that is typical of the protostomes.
In deuterostomy, radial cleavage occurs, whereby the blastomeres are arranged along a central axis and is characterized by several tiers of cells stacked on top of each other. Cleavage involves splitting into multiple cells called blastomeres, and results in a dense ball of these cells called a morula. In order to develop, the zygote undergoes a process called cleavage. Deuterostomyĭuring embryonic development, the fused gametes from the male and female-the sperm and the egg-form the zygote. They are a sister-clade of the Protostomes, and the two together with the Xenacoelomorpha form the major group of animals called the Bilateria-a major group animals which display bilateral symmetry and are mostly triploblastic. The Deuterostomes are a clade of animals that undergo deuterostomy during their embryonic development.
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