I was really stunned when I took off the top of the shell to see an obviously formed beak and head. This embryo was well developed, on the verge of hatching when the nest got depredated.
Eventually, I came to the last remaining bit of the yolk sac.
Here you can see the blood vessels attaching the embryo to the egg yolk:
To show a comparison of size, I put the embryo next to a penny and then next to the body of a dead fledgeling we found outside our house (it was probably run over). It's incredible to me that a newly hatched nestling goes from such a small size (around 6 grams) to fully grown in just under a month. Just two weeks after hatching, at the time the nestling leaves the nest, it has grown from 6 g to about 50 g. That's a staggering growth curve. No wonder those parents are foraging so frantically, they have to provide their young with a whole lot of food.
I've always wanted to someday take a class in embryology. My favorite part of the Skulls exhibit at the California Academy of Science was the skulls of fetuses. Taking apart this egg that contained an almost fully formed embryo just convinced me further that I should take a class in embryology, preferably sooner rather than later.
(For those of you who are wondering, Eagle Eyes and I think it is strangely fitting that we have all these bodies of robins in various stages of development right outside our front door. The neighbors haven't said anything yet. Who knows what the other members of the bird crew think, but they haven't said anything either. My advisor did comment as we left for the concert tonight that it was a bit odd no one had scavenged the fledgeling yet but that something had eaten part of one of the other robin's eggs I left lying out there).
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