Growing up, one thing that 8-year old Dana just couldn’t wrap her brain around was how chickens produced eggs, and more importantly, how eggs live outside that hen. 

I understood how eggs came home from the grocery store, were put in the fridge, and were the key components of tasty omelets (my dad makes a mean omelet). But what is the difference between grocery store eggs and a chick pecking its way into the world? 

First, let’s briefly touch on where things are inside that hen (poultry anatomy). Female chickens have two major parts to their reproductive system: the ovary and the oviduct. The ovary is a cluster of sacs attached to the hens back about midway between the neck and the tail. It is fully formed when the female chick hatches from her egg and contains several thousand tiny ova, each ovum within its own follicle.

Next, we’ll talk about how an egg goes from inside a hen to be a true egg. As the hen reaches maturity, these ova develop a few at a time into yolks. On the surface of every egg yolk, there can be seen a tiny, whitish spot. That spot is quite important. I’ve seen that white-ish spot (blastodisc) on the yolk in bowls on my kitchen counters and in science labs during experiments about developmental anatomy. 

That spot is actually a single female cell. If sperm is present when a yolk enters the infundibulum (part of the oviduct closest to the ovary), a single sperm penetrates the blastodisc, fertilizing it, and the blastodisc becomes a blastoderm. That blastoderm is our true egg. 

Now we have an egg that can develop into a chick. Without that fertilization, we would have an unfertilized egg (those were the eggs that came home from the grocery store and went into omelets).  That yolk, completely formed in the ovary, will rupture the follicle, then move into the oviduct. Other parts of the egg-like the shell, albumen, and chalazae, are added around the yolk as the yolk methodically moves down the oviduct. That hen has then made a complete egg, which is then laid.

After eggs are laid, they should be set, which includes storing eggs for at least three days to prepare them for incubation. Ideally, the next step should be in an incubator. Basically, a box that holds eggs while maintaining an appropriate temperature, humidity and oxygen level (55°F - 65°F and at 70 - 80 percent relative humidity) around the egg. 

Now we get to the fun part. The chick embryo uses oxygen and produces carbon dioxide, just like you and I. If you and I have lungs and a mouth to breath, how do those chicks, inside a hard eggshell, breathe? This gas exchange, or breathing, is insignificant during early incubation, but as the chick grows and develops, it consumes more oxygen and releases more carbon dioxide, as you’d expect. The eggshell that we see, and often crack on the side of a bowl before breakfast, is actually porous (just like a sponge is filled with pores). 

There are about 7,000 pores in a chicken eggshell. Carbon dioxide and moisture are released through the pores and replaced by gases from our atmosphere, like oxygen.

The final stage is when chicks hatch out of the shell. This is the point where eggs should be moved to a hatcher. Don’t forget about keeping those pores clear to maintain air passage to that chick. Hatching requires tremendous effort by that chick; it will be very active and then takes long rests. 

Rest assured it is all part of the process. This stage may take 10 hours to 20 hours as the chick battles its way into the world. Once chicks successfully leave the shell, increase the hatcher’s ventilation and leave them in it for about 24 hours or until their feathers are dry. They did it. Now they are ready to join your flock of happy, healthy birds.

 For more information about backyard poultry visit the UMN site

Dana Adams  is an extension educator -livestock at the University of Minnesota Extension Stearns, Benton and Morrison Counties.


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