That is, I know (or think I know) that the developing embryo uses the nutrients in the yolk for growth and metabolism. But…how? Is there an analog to the mammal’s umbilical cord connecting duck to yolk? Does the embryo absorb the nutrients transcutaneously?
My six year old demands an answer, and her children’s books and encyclopedias are not helping. 
Actually the yolk is what becomes the duck/chicken/any other bird. The albumen (white) is what provides the nutrients…but as for the actual meat of your question I have no idea…but I think that in early stages when it’s just a mass of cells diffusion gets all the nutrients where they need to go…i imagine once organs start to form it can breath and or ingest the albumen for nutrients.
Edit:i know theres a part of the albumen called the chalaze (sp?) That sort of supports the yolk/bird embryo so maybe nutrients can flow through it like an umbilical cord. Paging Colibri…
As the chick develops, the yolk shrinks, as it is what contains the nutrients. It does seem to be attached at the “belly” of the developing chick.
So…so…do ducks have belly buttons? :eek:
Ah…now that question actually got me somewhere!
Although another person over heresays no, there’s no scar as such because the end of the yolk sac is absorbed into the embryo before hatching.
So, still not sure if ducks have belly buttons, but it does seem clear that the nutrients from the yolk enter the developing embryo’s body through an attachment at the belly, very similar to placental mammals.
Neat little NOVA video here about duck development - it explains that the yolk contains everything the duck will need to build a body and the circulatory system helps get it to the right places.
Oh, awesome! Thank you, I’ll show it to my daughter, she’ll love it.
She’s all kinds of grossed out by the idea of a cloaca now. “Mama, they poop and pass their eggs through* the same hole*?!” 
Huh…I thought the whole yolk became a bird, but I guess it’s just a tiny piece in the yolk?
So if the yolk is the nutrient, what is the albumen for?
Do birds poop before hatching, or do they hold it? No wonder they try so hard to get out of the egg.
You know how when you crack an egg into a bowl there’s a little white dot floating on the yolk? That’s the actual ovum, which if fertilized, contains all the DNA; that’s going to become a bird through cell division and specialization. The yolk, lots of fat and protein, is used for the “building blocks” of the embryo, so you’re not exactly wrong to say it becomes the bird. But it isn’t part of the single cell that divides over and over to make all the bird’s cells.
The egg white is there as a cushion. Most sources say the it is also used to nourish the embryo; it doesn’t have fat, but it does have protein and water.
Oy, and now I just realized I don’t know how the WHITE gets into the embryo!
ETA: levdrakon, I don’t know if bird embryos excrete in the egg, but I would assume so. Certainly mammals do - the urine put out by a fetus is mostly from the amniotic fluid that they swallow.
Everything is added on the way from the ovary to the oviduct down to the cloaca. The oviduct has different parts with different glands, specialized on adding different egg components, including the albumin and the shell.
WhyNot, tell her the cloaca is actually like a big, big hallway entrance, and it has three different doors (GI/poop, kidneys/uric acid, and oviduct). Technically, the egg is inside the cloaca/hallway for a short period of time before being laid outside. Most of the development occurs in a tube that’s separate from the GI/poop tube.
Ah, thank you! I think that will help. She’s a little - ahem - shellshocked.