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Sometimes, in science, you just have to digitally reconstruct the genitals of a thinly sliced, 100-year-old embryo from an obscure New Zealand reptile, because you really want to know if your penis shares a common evolutionary history with those of crocodiles, birds, and snakes.
Spoiler: it does.
Among us back-boned animals—vertebrates—penises vary considerably, not just in their size and shape but in their presence, absence, and number. Male mammals have just the one, as do crocodiles, turtles, and some primitive birds like ostriches and ducks. Snakes and lizards have two ‘hemipenes’ (although they only use one at a time). And the vast majority of birds have none at all.
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Despite this variety, all these groups start out in much the same way. As embryos, they have a pair of genital swellings—two groups of cells that sit next to the ones that eventually produce the hind legs. These swellings get bigger, and merge in groups with a single penis, while staying separate in groups with hemipenes. In the penis-less birds, they do nothing.
To resolve this debate (or, at least, work towards a resolution), Thomas Sanger from the University of Florida turned to the tuatara. This spiny-backed, forearm-sized creature from New Zealand looks remarkably like a lizard, but isn’t one. The adults have no penises, and Sanger wanted to know if the embryos have a pair of genital swellings.
Read full, original post: Resurrecting a Set of Hundred-Year-Old Embryonic Genitals
