Why are eggs divided into yolks and whites?

[Pardel-Lux]

Continuing the discussion from Is the ratio of yolk/white basically the same across egg sizes? How about species?:

Good question! No idea. I hope this is not a hijack, but I would like to know what the whites and the yolks are for in the eggs, and this seems the right place to ask. I guess that the yolk has more fat and cholesterol (and water) while the whites have more protein (and water), but why are they separated like this, evolutionary speaking? How does a developing chick use what part of the egg and why? And I hope it is not a hijack because if there is a reason for that we might get an idea of what the ratio boundaries between yolks and whites are. Just like the form of an egg is quite optimal (try holding an egg between your index and thumb on the pointy and the round sides and squeeze hard, let’s see if you can break it) I guess the interior is quite optimized too. I would not be surprised if the ratio boundaries where relatively narrow, but the reason could be interesting.
ETA: I recently learned that threads can be spun off from other topics, so this is what I just did in a lazy way (copy & paste).

[markn_1]

I’m not sure if this is exactly what you’re asking, but as I understand it, the yolk provides most of the nutrients for the growing embryo. The white is mainly protection for the yolk. The white is 90% water without much in the way of nutrients.

[DSeid]

The yolk is more concentrated nutrition (the white is much more water and virtually no fat) and is surrounded by the yolk sac (humans have yolk sac in early embryologic development too BTW). The yolk sac actively absorbs and processes the nutrition for the developing chick embryo/fetus for most of its development.

Temporal transcriptome analysis of the chicken embryo yolk sac - BMC Genomics

My assumption is that in the final days the chick actually eats the white and digests it itself. Not positive of that though. If some of it is left behind less of a waste than leaving any yolk behind.

[crowmanyclouds]

I think this (if you squint a bit) works;
shell = uterus
yolk = placenta
white = amniotic fluid

[puzzlegal]

I think the yolk sack is equivalent to the placenta, and the yolk is, um, analogous to the nutrients transported by the blood in a mammal. I agree that the white is like amniotic fluid and the shell is kinda the uterus.

[DSeid]

So why this seems so interesting to me I don’t know but …

The egg white, aka the albumen, is full of a bunch of stuff to protect from infection. Much of the protein in it gets transferred to the yolk in early development and is then absorbed from there. Apparently most of the protein of the hatched chick comes from there.

6ed00d04959156ba42dcd28b857689697c65.pdf

Nutrient recovery from albumen and yolk is largely commensurate with body assembly through to comple- tion of the embryo by 14 d. Remaining albumen mixes with the amniotic fluid and is orally consumed until initia- tion of emergence. A portion of the albumen is absorbed by the small intestine to expand body glycogen reserves. The residual not absorbed contains digestive enzyme con- tributions and enters the yolk sac through its stalk at the jejunum and ileum. Interaction of the albumen-amnion digestive enzyme mixture with yolk sac contents leads to diverse alterations that influence subsequent use of lipids.

So confirmed that the chick just eats most of the last bits of the white, but, of course! it is more complicated than that.

Embryo expansion within the amniotic sac together with its physical movements initiate preparations for emergence. Albumen previously compartmentalized at the small end of the egg has its seroamniotic connection rupture (Feher, 1984). Free albumen can now enter the amniotic sac and form a composite of both contents. Pres- sure differential created by a continuing embryonic growth enhances an oral consumption of this mixture that passes through the gastrointestinal system (Oegema and Jourdian, 1974; Sugimoto et al., 1999). Partial absorp- tion of proteins from the albumen-amniotic mixture oc- curs by enterocytes capable of macromolecular absorp- tion during transit through the duodenum and jejunum. Antitrypsin factors strongly inhibit digestion, even though a full complement of pancreatic enzymes is pres- ent (Holdsworth and Wilson, 1967; Marchaim and Kulka, 1967; Yoshizaki et al., 2002). Albumen, together with the pancreatic enzyme composite, enters the yolk sac through its stalk with the aid of antiperistaltic activity of the colon (Sugimoto et al., 1989; Bryk and Gheri, 1990). Recovery of macromolecules during passage through the duodenum and jejunum is parallel to that of colostrums with postpar- turient mammals. Chick embryonic villi also have unique enterocytes that are capable of fluid and macromolecular transfer to greatly expand the vascular system with the appearance of albumen proteins. Such consumption and absorption is continuous until the albumen-amniotic fluid disappears and internal pipping begins.

[Darren_Garrison]

@puzzlegal

I think the yolk sack is equivalent to the placenta

No, I think the allantoic sac.

[jnglmassiv]

I caught this awesome BBC eggs programme [sic,lol] with David Attenborough a few weeks ago on our local PBS channel. Full recommendation!

BBC Two - Natural World, 2018-2019, Attenborough's Wonder of Eggs

[Pardel-Lux]

Unfortunately your link is not open in continental Europe, it looks very interesting indeed. I’ll have a look, perhaps I can get it via another channel.
Thanks a lot, @DSeid, your pdf is great*. And goes to show how very complex and fine tuned an egg is, which leads me to infer that concerning the original question from which this thread was spun the balance between so many and complex interactions implies that the ratio between yolk and whites and all the rest must be pretty constant for each bird species, and probably still quite constant too between species.
*And what a lot of fancy new words for me!

[DSeid]

@Pardel-Lux

implies that the ratio between yolk and whites and all the rest must be pretty constant for each bird species, and probably still quite constant too between species

But while a reasonable inference such turns out not to be the case between species. I posted the article about that in the mommy thread.