Today's an exciting time in the incubation process, because the embryo and its blood vessels are becoming large enough that they can sometimes (but not in every case) be seen in candling. They look a lot like a spider.
If we opened the egg today we'd see the embryo standing proud of the yoke and, if we could look at it under a microscope, it would show that it's turned onto its left side.
The incubated egg at day 4.
The heart has continued to grow at such a rate that it was starting to get squashed by the body. In order to give it more room to develop the embryo turns so that its left hand side is lying on top of the yolk. The heart is not yet enclosed by the body and if you opened the egg at this point in incubation you might even be able to see it beating.
The mouth, tongue and nasal passages start to develop today, as do all the other internal organs.
By the end of today all the organs the chick needs to sustain life when it hatches are present in their very early stages of development.
One of the fascinating things about candling chicken eggs is that the pattern of growth looks remarkably similar in different. At this point it looks a bit like a spider, with the embryo as the spider's body and the blood vessels its legs.
However, it isn't always very obvious and it's much easier to see through light-coloured shells than others. If you have Marans' eggs in your incubator, for example, you won't see anything at all at this point - the eggshell is too dark and the embryo isn't clear enough.
So you can see the development more clearly I've altered the contrast on the picture - the embryo and blood supply should be much clearer to you now.
If you decide to candle your eggs at this stage, this is what you're looking for - but if you can't see it yet, do not panic! It's still very early in the incubation process.
I know what you'll want to be doing - candling. It's so tempting, particularly when you see images like the one above.
But I'm going to suggest you wait to candle until, at the very earliest, tomorrow (day 5).
For two reasons. Firstly because it's not always obvious at this stage whether the embryo is developing or not. There's a danger that, if you candle now and you can't see anything, you'll panic and create unnecessary anxiety for yourself.
And secondly, because in these early stages of incubation the embryo's development is really quite fragile. Moving it unecessarily can cause it to stop developing.
So, leave your eggs where they are; keep turning them, make sure the temperature and humidity levels are stable, and look forward to tomorrow.
"The electricity supply to my incubator was cut for a couple of hours. Will the eggs be OK?"
They should be fine. A broody chicken would get off her nest for a while during the day anyway to eat and drink. Incubators like the Brinsea Mini Advance mimic this by cooling for an hour or so in each 24 hour period.
Eggs can withstand drops in temperature far better than they can deal with a rise in temperature.
If your electricity goes off for any longer than a couple of hours, keep the warmth in by covering your incubator with a blanket or quilt, leaving a gap so air can still get through. The likelihood is all will be fine.
On each day of this incubation series I feature my own chicks. These are two Wyandottes, one of them two weeks older than the other. See what a difference in size two weeks makes? Chicks grow very quickly. And notice the wing feathers developing on the older one.
Have you joined my incubating and hatching group yet?
If you're having a go at hatching your own eggs and it's new to you, you might want to consider joining my free 28-step series called "Hatching Happy Chickens".
You'll have e-mails personally delivered every day describing that day's developments in the incubation process, and pointing out as this page does exactly what you need to be doing.
It's free, and it's fun! Don't miss out - click on the pic to have a look at more information about exactly what you'll get.
The easiest way to follow my incubation and hatching series is to sign up for the e-mails. However, if you'd rather not do that, these are links you need to work logically through the series.
Important: copyright warning.
'The Incubated Egg' image is a commissioned artist's impression and, like all images on this site, is subject to copyright under the Copyright Law of the United States of America 1976.
Under no circumstances is permission granted to copy or otherwise use this image.
All other images of candling on this and other pages are my own and are not to be used without permission.
If you wish to use these or any other images on this site for educational purposes you are required to contact me by using the form to be found on this page.
For my full copyright policy see this page.