Several anatomical variations exist, relating to the embryological development of the two pancreatic buds. Neuroendocrine tumors, despite differing embryological origin, have common phenotypic characteristics. Until recently, phylogenetic reconstruction has been through anatomical (particularly embryological) similarities. The early embryological stages are reminiscent of ancestral gastropods and extant Monoplacophora. Studies based on anatomical, embryological, and paleontological data have produced different "family trees". The embryological origin of granulosa cells remains controversial. Congenital urethral diverticula can arise from several embryological sources. The junction between the embryological foregut and midgut lies just below the major duodenal papilla. Septo-optic dysplasia is a developmental disorder resulting from a defect of normal embryological development. Differentiation between the sexes of the sex organs occurs throughout embryological, fetal and later life. These tissues come from different embryological precursors and have distinct prenatal development paths. There has been debate on the embryological origin of cephalopod appendages. During embryological development, the thymus migrates downwards, dragging the inferior glands with it. He gave accurate descriptions of the ovoviviparous embryological development of the hound shark "Mustelus mustelus". This is a testament to their shared embryological origin from the first pharyngeal arch.