The body of animal form as part of an inherited developmental sequence and the sequence starts after fertilization of an egg produced by the ovary form a Zygote. And this Zygote is single big cell until the process of cleavage started dividing itself into a large number of smaller cells named Blastomeres.
The cleavage starts in several different patterns: cnidarians and sponges lack a distinct cleavage shape, but bilateral animals, exhibit either radial or spiral cleavage. Based on the Biologist, assumed that these two types of cleavage each evolved and the animals with spiral cleavage form a monophyletic group, compared to the animals with radial cleavage.
Radial cleavage are typical of echinoderms, chordates and hemichordates. It occurs with regulative development where each blastomere of the early embryo.
Spiral cleavage are typical of mollusks, annelids and other protostomes. It occurs in form of mosaic development, which the organ-forming determinants in the egg cytoplasm etc.
References:
www.wikipedia.com
Animal diversity 5th Edition.
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