One of the hallmarks of deuterstome development is that early cleavage divisions are radial. In contrast, protostomes typically display spiral cleavage. While these classifications are certainly huge generalizations, they nevertheless point to some of the fundamental differences between different phyla regarding early development. Play the movie at the below right to get a sense for the difference between these two cleavage patterns in two generalized embryos.
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Differences in cleavage orientation in idealized spirally and radially cleaving embryos