Urmund

In embryology , the opening of a developing embryo is called the primordial mouth or blastopore , where the surface of the body turns into the primitive intestine .

Most multicellular animals go through an essential phase, gastrulation , in the first steps of their embryonic development after the formation of the blastula or (in mammals such as humans) the blastocyst . As a result, the germ becomes a structure with an outer layer of cells, the ectoderm , and an inverted inner layer of cells, the endoderm . The primitive body has thus gained an inner surface on its outer surface, the indentation of which represents the primitive intestine . The place where the primitive intestine opens outwards is called the primordial mouth , because in many living beings the mouth develops from this area. Among the bilaterally built animals ( bilateria ) it is the primal mouths or protostomians ; in their further development a second opening of the primitive intestine is formed for excretion. The protostomes include, for example, insects .

In the deuterostomies or new mouths, on the other hand, to which all vertebrates belong, the primary opening of the primal mouth later functions as an excretory opening or anus , and the secondary opening at the other end of the primal intestine becomes their (new) mouth . In some basal animals such as the jellyfish , the primal mouth later serves both for intake and for excretion.

The distinction between primordial mouths and new mouths in the two-sided animals is an important aspect of the systematics of animals , because the similarity of the early stages of development indicates common ancestors and thus a relationship of species.