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The embryological origin of the mouth and anus is an important characteristic, and forms the morphological basis for separating bilaterian animals into two natural groupings: the protostomes and deuterostomes. In animals at least as complex as an earthworm, a dent forms in one side of the early, spheroidal embryo.
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.
[4] [1] It is now known that the fate of the blastopore among protostomes is extremely variable; while the evolutionary distinction between deuterostomes and protostomes remains valid, the descriptive accuracy of the name protostome is disputable. [1] Protostome and deuterostome embryos differ in several other ways.
Enterocoely is the stage of embryological development of deuterostomes in which the coelom forms. [4] This type of coelom formation occurs in deuterostome animals, which for this reason are also known as enterocoelomates. By contrast, in protostomes, the body cavity is often formed by schizocoely. [4]
Bilaterians can be divided, based on events that occur very early in embryonic development, into two groups called protostomes and deuterostomes. [20] Deuterostomes include vertebrates as well as echinoderms and hemichordates (mainly acorn worms). Protostomes, the more diverse group, include arthropods, molluscs, and numerous types of worms ...
Animals called protostomes develop through schizocoely for which they are also known as schizocoelomates. Schizocoelous development often occurs in protostomes, [1] [5] [6] as in phyla Mollusca, Annelida, and Arthropoda. Deuterostomes usually exhibit enterocoely; [7] however, some deuterostomes like enteropneusts can exhibit schizocoely as well ...
A cell can only be indeterminate (also called regulative) if it has a complete set of undisturbed animal/vegetal cytoarchitectural features. It is characteristic of deuterostomes—when the original cell in a deuterostome embryo divides, the two resulting cells can be separated, and each one can individually develop into a whole organism.
Neural crest cells enter these arches where they contribute to craniofacial features such as bone and cartilage. [5] However, the existence of pharyngeal structures before neural crest cells evolved is indicated by the existence of neural crest-independent mechanisms of pharyngeal arch development. [ 6 ]