part of develops from adult male reproductive system vas deferens FBbt:00004968 Paired tube of the male genitalia which opens into the anterior ejaculatory duct. Its distal portion is enlarged, forming the seminal vesicle. The twisted narrow portions of the vasa deferentia merge just before the junction with the anterior ejaculatory duct. In recently emerged flies, three distinct wall layers of the vas deferens are visible: a thick inner epithelium, with columnar cells containing small nuclei, a thin inner sheath with inconspicuous, widely spaced and flattened nuclei and an outer sheath of finely reticular cytoplasm containing an occasional large nucleus. fly_anatomy.ontology male-specific anatomical entity multicellular structure presumptive vas deferens