If we opened the egg at this stage of incubation we would see the embryo now has a developed blood supply which is reaching throughout the yolk. The embryo itself is fully curled into the shape of a letter 'C'.
If we used a specialist chicken egg embryo heart monitor, the heartbeat would be strong enough to register today.
At this point the embryo's heart will be beating at the rate of between 260 and 280 beats per minute. (Source : Avian Biotech International).
The force of the heartbeat now causes the embryo to move quite freely within the egg - sometimes you can see movement when candling at this stage, although it's more common later in the process.
The incubated egg at day 6.
The bone structure of the chick is now developing; at a point mid-way between today and tomorrow the wing and the legs are able to bend at the knee and elbow.
The beak also begins to grow although at this stage the egg tooth, which will help the chick 'pip' through the eggshell, is not yet present.
At a more technical level the chorion and the allantois come together today to form the chorioallantois or chorioallantoic membrane. It's full of blood vessels and surrounds the embryo - in human embryos this is the placenta.
So far, the chicken embryo has used its own blood system to provide it with oxygen but as it grows, that's no longer enough. The chorioallantois is immediately next to the shell and through its blood supply helps provide oxygen to the developing chick, and takes away waste.
With a light coloured shell you may be able to see some of the features in the picture above. The blood vessels are clearly larger than they were yesterday and for the first time, the embryo's eye can be seen.
Below is the same picture but with the colour and saturation levels changed so that you can see more clearly. In this image the way the blood vessels are developing branches right through the yolk is clear and the embryo in the centre of the blood supply, in the shape of a letter 'C', is also very obvious.
Below is the same image but I've added annotations to the blood vessels, embryo, spine, and albumen or white of the developing egg to make sure you know what you're looking at. The yolk is the darker area immediately above the albumen.
In the picture below I've taken out the colour to try to highlight the eye of the embryo more clearly. It's the darker spot right at the end of the arrow.
At this stage of incubation it's unlikely that you will see this level of detail with the naked eye.
The usual - keep an eye on the incubator's temperature level to make sure it's stable. If you're using an incubator which requires you to top up the water levels manually, keep watching it to make sure the humidity level remains around 45%.
It's tempting to candle but, as you can see, it's not always easy to see what's going on in the egg at this point. Wait until tomorrow.
The developing embryo is most vulnerable during the first week of incubation. It's not unusual for an embryo to stop developing during this period.
Potential reasons for "early death" are many : the chickens the eggs came from might have been old, diseased or too heavily medicated; eggs may have been stored too long or not in the right conditions (see here for tips on how to store fertile eggs before they go into the incubator); they may have been dirty and carrying bacteria which has leached through the shell; the temperature in the incubator may be too hot or too cold; the eggs may not have been turned frequently enough.
To give your eggs the best chance of hatching successfully, make sure you look after them well even before you set them in your incubator.
Is it too early to candle my Black Copper Marans eggs?
Black Copper Marans' eggs like this one are very dark and one of the most challenging to candle.
Yes - it's very hard to see inside one of these eggs, even with a high intensity candler, because the shells are so dark.
The best way to deal with them is to wait until day 10 and look for the air cell, which by that stage should be fairly large.
At each new stage of this incubation series I feature a photo of one of my own chicks.
It helps us keep our minds focused on what we're working towards - a healthy, happy chicken.
This is one of my Golden Laced Wyandotte chicks when she was three weeks old. She still has her downy chick fluff but you can also see her feathers coming in towards the bottom of the pic.
(If you're not sure what 'lockdown' is don't worry - we're covering that later in this series).
So - do you have any questions for me? Or any comments you'd like to make? If so - feel free to leave them here. I will usually get back to you within 24 hours.
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-day 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 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.
'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.