Anatomy of an Egg

by Deb on March 30, 2010

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More on the easter theme and just for a bit of fun, I thought I’d talk about what’s in eggs and what it all does.  Eggs have to be the equivalent of the uterus and placenta, so there’s a lot packed into them.  What an opportunity to crack an egg and have a look at all the parts with your kids.

From University of Illinois

To start at the outside and work in:

Shell:

The shell is mostly made of calcium carbonate, the same as sea shells, and can be white or brown depending on the breed of chicken (or of course green, blue, speckled, etc if we’re talking about other birds).  It’s job is to keep everything in and keep bacteria out, but it isn’t a completely closed system.  There are thousands of small holes or pores that allow for some moisture and carbon dioxide movement and to form the air cell.  They are mostly on the large end, and are covered by a protective coating called the cuticle.

Shell Membranes:

There are two shell membranes, inner and outer.  This is the white ‘skin’ inside the egg, and the air cell forms between them.  They are equivalent to the amniotic membranes in mammals and are so important that reptiles, birds and mammals are called ‘amniotes.’  Basically, fish and amphibian eggs develop in the water, when they crawled out and became reptiles then birds and mammals the amniotic membranes created their own personal little pool of water for developing embryos.  When an egg is first laid it completely fills the shell but as it cools it shrinks, which is where the air cell comes from.  In the days before hatching, a chick punctures the air cell and begins to breathe, it is partly the buildup of carbon dioxide from this breathing that triggers hatching.

Albumen:

There are two types of albumen, or white, the inner albumen is more solid, the outer is thinner, and as an unfertilised egg gets older the inner albumen tends to break down and become more watery.  You can see this when you fry an egg – a fresh egg will have more of a contained white with an inner oval around the yolk, with an older egg the white spreads out.  The albumen is made of water and protein, it is the personal lake in which the embryo develops.  This is important because liquids allow things to move easily, so the chemical signals the embryo produces can spread around it and wastes can be moved away, and they cushion and protect the yolk.  The protein also provides some of the building blocks for the embryo to turn into cells.

Chalaza:

Now you know what those annoying white, thick bits are, the chalazae (that’s not a spelling error, it’s the plural).  I don’t know if anyone else finds them annoying but I don’t like the texture.  They are like ropes that stretch between the yolk membrane and the shell membranes on either side and keep the yolk in the middle of its shock protecting lake.

Yolk:

The yolk is contained within a membrane and is mainly made up of proteins and fats.  The amazing thing is that it is all one cell.  Yep, the yolk is the actual ovum, one of the few cells big enough to see with the naked eye.  The fats and proteins are to make the chick, and when it is born the rest of the yolk ends up attached to its intestines and it feeds off them.  Kiwi chicks can actually live on it for 6-10 days.  This is the reasons eggs are so nutritious – they contain all the nutrients, vitamins and minerals needed to build a bird (or reptile).

Germinal Disc:

This is where fertilisation takes place, it is the entrance to a funnel into the yolk.  A sperm gets through the disc and swims into the middle of the yolk, which is where the embryo begins to form.

Blowing Eggs:

Now for the Eastery bit.  Blowing eggs allows you to keep the shells as long as you don’t break them, because they are calcium carbonate they don’t go off.

  • Hold the egg and shake it firmly to break up the yolk.
  • Hold the egg over a bowl and use a pin with a pearl head to gently drill holes in both ends.  Gently move it around to enlarge them a bit.
  • Use the pin or a toothpick and jiggle it around inside to further break up the yolk.  As you now know, it has a cell membrane surrounding it that needs to be broken.
  • Put it over the bowl, put your mouth completely over one end and blow hard.  Depending on how big you made the holes the chalazae and parts of the yolk may be difficult to get out, use the pin to pull them or make the holes a bit bigger.  An older egg will be easier to blow because the albumen has thinned down.
  • When you have it all out, put the egg under running water to clean it, then leave it to dry.
  • Decorate or use!
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