Protostomes and Deuterostomes

Protostomes includes organisms such as annelids, molluscs and insects. During protostome development the first opening to appear in a blastopore becomes the mouth of the organism. This pore deepens, forming the gut and eventually the anus. Protostomes exhibit spiral clevage in their cells, since each layer is offest slightly. Additionally the cells roles (ie: this cell will be a neuron) are determined very early on in development. The role is chosen, and stays as such in the organisms adult form. The coelom of the organism develops through a process called schizocoely, in which masses from the mesoderm migrate and form the coelom.

Deuterostomes include organisms such as echinoderms, hemichordates, chordates and vertebrates. Unlike the protostome, rather than forming the mouth furst during blastulation, the anus forms first. Furthermore, deuterostomes exhibit a radial clevage, as cells are not offset with each layer. These cells fate and role are also indeterminate. Rather the cells role in the body of an adult will be determined at a later time, usually though stem cells. The coelom of a deuterostome is developed through a process called enterocoely, where the mesoderm folds and pinches to form the coelom.