The archenteron is a indentation that forms early on in a developing blastula. The archernteron will go on to develope into either the animal's anus or mouth. The fact that this develops in two different ways is used for animal classification as either a Schizocoely or a enterocoely .

The archenteron is known as the primitive gut that forms during gastrulation in the developing embryo. The indentation that is actually formed is called in amphibians and fish, the dorsal blastopore, and in birds and mammals it is called the primitive streak.