Embryological origins of the mouth and anus

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. This dent, the blastopore, deepens to become the archenteron, the first phase in the growth of the gut. In deuterostomes, the original dent becomes the anus, while the gut eventually tunnels through the embryo until it reaches the other side, forming an opening that becomes the mouth. It was originally thought that the blastopore of the protostomes formed the mouth, and the anus was formed second when the gut tunneled through the embryo. More recent research has shown that our understanding of protostome mouth formation is somewhat less secure than we had thought.Acoelomorpha, which form a sister group to the rest of the bilaterian animals, have a single mouth that leads into a blind gut (with no anus). The genes employed in the embryonic construction of the flatworm mouth are the same as those expressed for the protostome and deuterostome mouth, which suggests that the structures are equivalent homologous, and that the older ideas about protostome mouth formation were correct. An alternative way to develop two openings from the blastopore during gastrulation, called amphistomy, appears to exist in some animals, such as nematodes.

Humans, unlike the protostomes and other deuterostomes, already have a formed gut tube at the time of formation of the mouth and anus. In humans, the mouth is formed first, during the fourth week of development, and the anus is created four weeks later, temporarily forming a cloaca.

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