Inside the Egg


Amazing Transformation
Look at this crane egg. Until the chick started punching a hole in it, the outside looked nearly the same, day after day, for about 30 days since the egg was laid. But on the inside, this egg has gone through an amazing transformation! Let's learn about what has been happening inside the egg, and then we'll ask you to close your eyes and imagine what it's like to be a whooping crane chick inside the egg.

Step by Step
The day it was laid, the inside looked like the inside of any chicken egg we buy in a store. Yellow yolk was surrounded by clear albumen (also called the egg white), all in a thin membrane just inside the shell. One tiny speck on the yolk was the only sign that the egg was fertile. That speck was the chick at its earliest stage of development.

The Yolk
The yolk of the egg provides nutrition for the developing baby. Depending on the species, the yolk can be large or small relative to the size of the egg. Songbird eggs are about 1/5 yolk; duck eggs are about 1/2 yolk. For all birds, the yolk is way too big to ever fit inside the chick's growing digestive system. Instead, the developing chick's digestive system is directly connected to the yolk by a tube.

The Albumen
The albumen is entirely outside of the chick. Its job is to cushion the chick. It must also protect the chick from drying out, give structural support to prevent the yolk from getting flat, and transfer water and nutrients to the yolk as the chick develops. But the chick isn't actually in contact with the albumen; the chick is inside a little sac of fluid called the amniotic cavity.

The Chick
A chick in early development has no senses at all. As the chick's body takes shape, its sense organs develop little by little. It soon becomes aware of sounds. Chicks can hear their parents' voices inside the egg. That's how some songbirds learn their song. When a chick pips a little hole through the inside membrane of the egg, it gulps in a bubble of air before it even pokes a hole in the shell. That little airsac allows the chick to start breathing before it hatches, and also allows the chick to make its first sounds. In many species, chicks listen to their brothers and sisters in the eggs. Parents sometimes anticipate hatching by listening to the peeping sounds made by the chicks for the last day or two before hatching.

Read-Aloud: Visualize This!
Now close your eyes and listen to this description: Imagine being inside a crane egg. The front of your body is curled around the yolk, which is attached to your stomach by a tube so you never feel hungry. Your backside is snuggly pressed in by the albumen. Even though you're floating in liquid, you can hardly move. As you get close to the time of hatching, your eyes start working and you probably notice that it gets a bit brighter every now and then, but you don't realize that it happens when your parents are off the nest. Your ears start working, too. You may learn what your parents' voices sound like. And if you're a whooping crane chick in Operation Migration, you hear recordings of an ultralight plane engine. Inside this warm, cozy egg, the noise of an ultralight isn't the least bit scary and you grow used to it.

As days go by, you suddenly need to scratch, but it's not your body that itches. You simply have an uncontrollable urge to scratch at that membrane inside the egg shell. Your body is so tightly pressed in that your beak is tucked under your right wing. You scratch and scratch, but you don't know why. Suddenly, you punch a hole in the thin membrane, and you can breathe! Whew!

Now the urge to scratch is even stronger, but this time you're scratching at the hard egg shell. This is not nearly as easy. You tap and tap for a whole day before you finally break through. This hole is called the "star pip." Now fresh air can get in, but you're exhausted! You fall asleep and don't wake up for about 24 hours.

You wake up with a new feeling of determination to make that hole bigger. You can't whistle while you work, but you can peep. Peeping and scraping, pecking and peeping. Soon the little hole is as big as a dime (not that you know a thing about money!). You can rest a little, but you spend most of your time working, chipping away chunks of the wide end of the egg. To reach more places, you struggle to twist your body around. This is a hard job, but something deep inside makes you keep working until it's done. And you're stronger, breathing fresh air, so you keep at it until in an hour or so you've done it! The big end of the egg is off, and you wiggle out in an exhausted heap. Happy birthday!

Crane egg photo by USGS Patuxent Wildlife Research Center

drawing adapted from Keeton, Biological Science, 1967
Early Development


drawing adapted from Cambell and Lack, A Dictionary of Birds, 1985
Two Weeks Before Hatching


Courtesy of USGS Patuxent Wildlife Research Center
In Hatching Position


Courtesy of USGS Patuxent Wildlife Research Center

Look at Listen!

Courtesy of Operation Migration



Try This! Journaling Questions

  • Why do you think the yolk of duck eggs is 50% of their egg compared to the yolk of a songbird, which is only 20%?
  • Why do you think the albumen turns white when an egg is cooked?
  • Why can't the chick make sounds before it breaks through the egg's inner membrane?
  • Why do you think they call the first hole in the shell the "star pip"?
  • Why do you think the scientists at Operation Migration play recordings of ultralight motors before the Whooping Crane eggs hatch?

First record your thoughts in your Science Journal, then see discussion.

National Science Education Standards

  • Plants and animals have life cycles that include being born, developing into adults, reproducing, and eventually dying.
  • Organisms have basic needs. They can survive only in environments in which their needs can be met.