Bilaterians likely evolved from an ancestor that was radially symmetrical. There have been suggestions that the blastopore started out as the digestive surface on a radial organism that became elongated (and thus bilaterally symmetrical) before its sides closed over to leave a mouth at the front and an anus at the rear. The story is a little more complex, because the blastopore itself does not directly give rise to the mouth of these worms. In humans (a deuterostome), the development proceeds differently. The
buccopharyngeal membrane is created in the
foregut and is perforated during the fourth week of human development, creating the primitive mouth, whereas the
cloacal membrane is created in the
hindgut and is perforated during the eighth week of human development, creating the primitive anus after the mouth opening has already been created. ==See also==