For the next three days of incubation until hatch, the chick just carries on growing. The albumen (egg white) has now completely disappeared and today is the start of the yolk beginning to be absorbed into the chick's body.
If we opened the egg today what we'd see is mostly chick, with a large expanse of yolk. On top of the yolk is a by now very complex network of veins.
This area is going to be critical for the chick's well-being, and it's the reason why "assisted hatching" - helping the chick to hatch - is not advisable except in very exceptional circumstances.
We'll return to this subject nearer hatch time.
Our chick is continuing to turn slowly towards the blunt end of the egg where the air cell is. It's here the 'egg tooth' will first break through the inner membrane and then the shell.
For a picture of what an egg tooth looks like, have a peek at my "Chick of the Day" photo later on this page.
Here's a link if you want to see it now.
From now until the end of incubation there's not much change in the chick itself except that, as the yolk is retracted into the body, the chick grows in size.
The yolk will nourish the chick during its last days of incubation but will also be important in keeping it well nourished for the first day after hatching.
It's one of the reasons why it's possible for breeders and hatcheries to send day-old chicks by post - the chicks don't need food at that point.
We can't see anything much now.
The chick is filling most of the egg, with the yolk taking up the remainder of the room. Sometimes it's just possible still to see some large blood vessels at the bottom of the egg.
The air cell is quite large and will continue to grow over the next three days to hatching.
This is one of my Buff Orpington eggs - see the end result by clicking here!
This is an exciting - and a critical - time.
Today we need to prepare our incubator for hatching. After today we're into a waiting game when we will no longer candle and the incubator will remain closed.
You may have heard the phrase "incubator lockdown" - that's exactly what this is.
There are five things you need to do today to make sure your incubator is properly set for incubation.
To see what they are, you will need to click on this link to go to my page which deals with lockdown in detail.
It will open in a new page so that, when you've finished, you can come back here for the rest of today's information.
It's really, really important that you do read the "lockdown" page. The five steps it describes needing to be done today are critical for a successful hatch.
The time between now and hatch is a critical one.
It's particularly important that you candle all your eggs today - it's one of the five steps in my "lockdown countdown".
This is a picture of one of my eggs candled at Day 18. It's not easy to see anything because this embryo had developed perfectly normally until that point, so the chick was filling most of the shell and there's not much else to be seen.
Except ... look closely.
It has a crack.
Here's another picture with the image brightness turned up so it looks very grainy, but it highlights the crack. In case you still can't see it I've also annotated it.
To try to make it even more obvious, in this next picture I've altered the saturation of the image.
The crack is the thin line which now looks red. It starts at the tip of the bottom arrow and continues to the tip of the top one.
Once I'd candled it I examined it more closely and it had a tiny bit of gunky liquid starting to ooze out of it.
Had this egg been left in the incubator for the remainder of incubation it would have exploded and spread bacteria over all the other eggs in the incubator.
The entire hatch would have been compromised - and I had 23 eggs in that incubator.
Which is why it's so important to follow the procedures for lockdown. To the letter.
Most late embryo deaths and unsuccessful hatches are due to different forms of bacteria or loss of humidity at critical times. I was lucky - this was the only egg to be affected in this batch.
It's likely that I'd become complacent and not been careful enough about cleanliness when handling it.
As incubation progresses, the shell becomes more and more porous and therefore more susceptible to taking in external bacteria. So washing hands both before and after candling is extremely important.
If lockdown is properly carried out then the chances of anything going wrong are minimised. However, the last couple of days of incubation - and of course hatching itself - is a time when things can go badly wrong.
How to stop that happening?
If you've followed through with the five steps for lockdown and all my recommendations for cleanliness right from the start of this process, you've reduced the likelihood of bacteria entering the increasingly porous eggshell.
Your chances of a successful hatch are high.
If it has any signs of a sticky liquid coming from the crack, don't hesitate - get rid of it now.
It's heartbreaking to get this far and have to lose one I know but trust me - it's necessary.
If there's no liquid you have the choice of trying to seal the crack and hoping the chick is still alive. It's a risk, because if it's died the egg may explode. But it's been done successfully by several people I know.
The best things to seal a crack with are a very strong duct tape, or superglue.
We're nearly there - keep your eyes on the prize!
At each stage of this incubation series I feature one of my own chicks so that we bear in mind always what we're aiming for throughout these three weeks - hatching healthy, happy chicks.
This is Buffy, one of my little Buff Orpington chicks at just two days old. See her still in the egg earlier on this page - click here!
You can still see the egg tooth - the lighter tip to the beak.
Such a great feeling to know that our care and knowledge will help successfully hatch little chicks like this.
Are you excited for hatch day? I am!
Click on any of these images to go to that page.
These are links you need to work logically through my incubating and hatching series.