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In page Hemipenis:

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This theory proposes that the single unpaired penis is the ancestral state for amniotes, and that this trait was retained by most amniotes today. A look at the embryonic underpinning of hemipenes and penises of other animals suggests that there are fundamental differences in their developmental stages, particularly their origin of development relative to the embryonic cloaca. Specifically, the hemipenes of squamata are found to develop on the posterior side, while the paired genitals of non-squamata amniota develop on the anterior side.[1] This developmentally significant difference suggests that the two types of penises could have distinct homologies, and it is thought that this could be attributed to variance of signaling genes during embryological development.[1]