Mesoderm is formed from mesenchyme originating from the archenteron. The bottom of the archenteron is composed of macromeres. The gut's final length depends on cell rearrangements within the archenteron. The open end of the archenteron is called the blastopore. The inner wall will be called the archenteron; the primitive gut. This narrowing and lengthening of the archenteron is driven by convergent extension. The developing digestive tube of an embryo forms as a blind pouch called the archenteron. The endoderm of the archenteron will fuse with the ectoderm of the blastocoel wall. The original ball of cells develops a lateral pouch, the archenteron. The archenteron will open to the exterior through the blastopore. As the blastopore deepens, a new embryonic cavity develops, the primitive gut, or the archenteron.