After fertilization the egg begins to divide repeatedly, a process termed cleavage . In some animals, dividing cells of the embryo are offset from each other, a pattern known as spiral cleavage .
Spiral cleavage can be determinate - if cells are separated they form a partial embryo.
In other animals, the dividing cells of the embryo are aligned, a pattern termed radial cleavage . At this point the embryo is a clump of dividing cells that become hollow ball known as a blastula . One wall of this ball indents and grows inward. The opening is a blastopore . If a coelome forms but mesoderm lines only one side of the coelome, the animal is a pseudocoelomate and has a pseudocoelome .