Apr 26 2013
We’ve had eggs on our minds this week while we’re watching them hatch at the Cathedral of Learning peregrine nest.
Eggs start as the familiar objects we see every day in our refrigerators and miraculously become baby birds. The process is so amazing that I’m devoting two Tenth Page articles to it.
Shown above is the un-incubated egg we know so well. If fertilized before it’s laid — and then incubated — it becomes a bird. Each component plays a part.
- Blastodisc or germinal disc: Potential embryo. If fertilized and incubated this small circular spot on the yolk becomes a chick.
- Yolk: Food for the embryo. The female’s ovary deposits layers on the yolk to increase its size before ovulation. Yellow layers are laid on during the day, white ones at night, so the yolk has rings like a tree. It’s housed in a yolk sac which is why you have to “break” the yolk when cooking. The yolk is ovulated with the germinal disc attached (cradled by the yolk) so the food is next to the potential embryo even before fertilization. As the embryo develops, the yolk shrinks.
- Albumen = Egg White: Food, water, shock absorber, and insulation from sudden temperature changes. The albumen makes up 50% to 71% of the egg’s total weight. It’s laid on after fertilization while the yolk-with-germinal-disc rotates gently in the oviduct. As the embryo develops the albumen shrinks too.
- Chalazae: Because the yolk is rotating during albumen deposition, twists form in the albumen. Chalazae act like springs and stabilizers to keep the yolk and embryo in place inside the egg. They’re the white twisted bits in the egg white. (Totally amazing! Shock absorbers, insulation, springs and stabilizers!)
- Inner Shell Membrane: the first of two membranes that hold the embryo-yolk-albumen together
- Air Space: Between the inner and outer shell membranes the air space acts as a condenser for moisture exchange. This is where the baby bird takes its first breath before hatching.
- Outer Shell Membrane: The final packaging before the shell is laid on. It’s attached to the shell when you crack open an egg.
- Shell: The female’s uterus deposits calcium on the outer shell membrane to make the hard enclosure for the egg. The shell has microscopic pores to allow air exchange for the developing embryo.
- Cuticle: A thin layer on the shell that adds protection. The cuticle has caps on top of the pores that close when necessary to protect the embryo.
Eggs have the tools and potential to become baby birds. Next week I’ll show you how.
(illustration from Wikimedia Commons; click on the image to see the original. Today’s Tenth Page is inspired by page 420 of Ornithology by Frank B. Gill.)