TableĀ 1.

Variation among amniote phalluses. *, this study.

external genital swellings number and positionphallus number and positionopen sulcus or closed urethragerm layer contributing to sulcus/urethraprimary erectile mechanism
Sphenodonpaired, lateral buds*no mature phallus
Squamatapaired, lateral budspaired, lateral hemipenesopen sulcusectodermblood/lymph
Cheloniapaired, lateral budssingle, midline phallusopen sulcusendodermblood
Crocodiliapaired, lateral budssingle, midline phallusopen sulcusendodermmuscle/blood
Mammaliapaired, lateral budssingle, midline phallustubular urethraendodermblood
Aves (Palaeognathae, Anseriformes)paired, lateral budssingle, midline phallusopen sulcusendodermlymph
Aves (Neoaves, Galiformes)paired, lateral budsno mature intromittent phallus