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In protostomes, the coelom forms when the mesoderm splits through the process of schizocoely, while in deuterostomes, the coelom forms when the mesoderm pinches off through the process of enterocoely. Protostomes undergo spiral cleavage, while deuterostomes undergo radial cleavage.
In both deuterostomes and protostomes, a zygote first develops into a hollow ball of cells, called a blastula. In deuterostomes, the early divisions occur parallel or perpendicular to the polar axis. This is called radial cleavage, and also occurs in certain protostomes, such as the lophophorates.
Coelomates are further divided into one of two groups called protostomes and deuterostomes, based on a number of developmental characteristics, including differences in zygote cleavage and method of coelom formation.
The earliest distinction between protostomes and deuterostomes is the type of cleavage undergone by the zygote. Protostomes undergo spiral cleavage , meaning that the cells of one pole of the embryo are rotated, and thus misaligned, with respect to the cells of the opposite pole.
Learning Objectives. By the end of this section, you will be able to do the following: Explain the differences in animal body plans that support basic animal classification. Compare and contrast the embryonic development of protostomes and deuterostomes.
There are a series of other developmental characteristics that differ between protostomes and deuterostomes, including the type of early cleavage (embryonic cell division) and the mode of formation of the coelom of the embryo: Protosomes typically exhibit spiral mosaic cleavage whereas deuterostomes exhibit radial regulative cleavage.
Each cell at the two-cell stage can develop into a complete organism. This is not the case of protostome embryos, which display determinate cleavage. Protostomes and deuterostomes differ in their developmental program. Protosomes have spiral cleavage at the eight cell stage, while deuterostomes have radial cleavage.
