As mentioned before, "candling" is when you shine a light through an egg and observe the shadows to check the development (or lack of development) of the embryo inside. You can buy candling gadgets -- some are pretty cool but quite pricey for what they do -- but you can easily make your own with a cheap $2 LED flashlight as shown above.
The room needs to be completely dark. Just shine the light through the egg, cupping your hand around the edges so no light leaks through.
You can get a decent 9-light LED flashlight for about $2 at Tractor Supply Co or most auto parts stores. LED flashlights are good because they are small and compact, put out a lot of light, and don't heat up very much. You don't want to "cook" your egg while you're candling it!
Things to remember when candling:
- Handle eggs CAREFULLY!
- Keep candling sessions brief and limited to a max of once a day. Less is better. You don't want the eggs to cool off too much and you don't want too disturb the growing chicks anymore than necessary.
- Candling isn't so much an art as it is a comparison. If you aren't sure what you should be seeing, compare your egg day to photos (there are a ton online).
- The darker the egg shell color, the more difficult candling will be. We breed Marans, which are noted for having some of the darkest brown eggs of all of the chicken breeds. We usually can't see shadows of any kind until way late in the game, if at all. We also have Ameraucanas (blue egg layers) and have had some eggs that look like regular light blue eggs but when candled, the shells were so thick we might have seen better light through cardboard.
There is a lot of information out there about candling, so if you see something you aren't sure of, or if you aren't seeing anything and you think you should be, do a little research. Everyone has their own opinion, but if you aren't seeing any development at all after day 10 (and you have light colored eggs so you know you should be seeing something) this is when many people will start taking out and disposing of the "blanks". These eggs could have just not been fertilized, or they could have just been "quitters" for one reason or another.
Ok, so now on to Day 4:
- The embryo has almost completely risen off the yolk and is now starting to turn onto it's side
- Toes begin to form