Hydatids, as appendices of testis or epididymis, were discovered by Morgagni in 1703 and 1705 and published by him in 1761. Hydatids are considered to be remnants of the cranial part of the Mullerian duct (MD), Wolffian duct (WD), or mesonephric tubules. They are localized as sessile or pedunculated appendices at the cranial pole of testis and at the head of epididymis, or at analogous organs in women. The clinical relevance is known: acute scrotum with torsion of appendices, or metaplasia. However, little is known about the embryological development of hydatids. Therefore, we studied the origin and development of appendix testis (AT) and appendix epididymidis (AE) in human embryos from stage 14 (Carnegie Collection), 6.5 mm GL, 32 days, to fetuses of 170 mm, 17th week. Light and scanning-electron microscopy as well as plastic reconstructions from serial sections of the cranial parts of MD and WD reveal that hydatids already form during regression or transformation of the ducts. At stage 18, 15-16 mm GL, 44 days, the cranial parts of MD and WD exhibit morphological features that give a preview on the definite form and position of later appendices. In fetuses from 45 mm GL, ninth week onward, we found anlagen of pedunculated hydatids (AE) deriving from the ampullated cranial end of the WD, which in many cases opened into the coelomic cavity. The unpedunculated AT derived from the persisting funnel region of the MD. The development of duct-independent, accessory appendices was observed. We paid special attention to a pedunculated hydatid in a fetus of 120 mm, 14th week, and the cranial regressing WD. A classification of hydatids is presented. Photographs and histological sections of (sessile) appendices testis (AT), and (pedunculated) appendices epididymidis (AE) with torsion of stalks exhibit the final forms and positions of hydatids in adult.