Human yolk sacs
Mar. 22nd, 2008 09:17 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
mmcirvinPZ Myers talks about the evolution of placental mammals from egg-laying organisms, and what's left behind in the genes. Near the beginning, he talks about how human embryos still have yolk sacs.
That's actually something I've wondered about for a while. When Sam was pregnant with Jorie we didn't have an ultrasound that early, but my sister Megan had one pretty early and I remember her showing me proto-Greta's yolk sac. Turns out there's no actual yolk in it, not any more, but it's there. Ontogeny doesn't recapitulate phylogeny, but you can see how people might think that was the case since there are all these evolutionary leftovers in our development.
That's actually something I've wondered about for a while. When Sam was pregnant with Jorie we didn't have an ultrasound that early, but my sister Megan had one pretty early and I remember her showing me proto-Greta's yolk sac. Turns out there's no actual yolk in it, not any more, but it's there. Ontogeny doesn't recapitulate phylogeny, but you can see how people might think that was the case since there are all these evolutionary leftovers in our development.