The second week in the life of a chicken embryo sees massive development. It is, for me at least, the most exciting stage of incubation.
Between now and the end of this week the embryo will become more and more like a baby bird and will turn to make sure it's facing the right way for hatching.
If we opened the egg today we'd be able clearly to see the yolk, blood vessels, the embryo and in particular the eyes, which at this stage look massively out of proportion to the rest of the chick, and the air cell at the blunt end.
The incubated egg at day 8.
Did you know that chickens have three eyelids?
Ever wondered why, when you take a picture of a chicken, its eye often looks as though it's got some kind of curtain across it?
The third eyelid is called the nictating membrane and today it's developing in our chick. It's a transparent membrane which keeps the eye moist and free from grit by moving across it - not up and down like the upper and lower eyelids. It's clear so that the chicken can keep an eye open - literally - for predators even as it blinks.
The pterylae or feather tracts - the part of the chicken's body from which the feathers sprout - are developing today and the down the chick will be born with, known as natal down, is growing.
Today the blood supply continues to grow and the skeleton of the embryo is developing and hardening as the blood vessels provide calcium taken from the shell. In particular, the thigh bones start to fill with marrow.
It's for this reason that it is so important to choose eggs for incubating which are in the best condition, from hens which have been supplied with the correct amounts of calcium.
From now on it becomes more difficult to see as clearly as in the first week because the embryo is growing rapidly and the blood vessels are becoming more dense. By the end of the week we won't be able to see much more than a dark shadow.
Here's exactly the same picture but with the colour and saturation levels changed. Now the 'hot spots' of the embryo and blood vessels are much easier to see.
If you haven't yet candled your eggs you may want to do so now. Some people candle at day 7 (or even earlier); some leave it until day 10.
If you are lucky enough to be using a broody hen you may not want to candle at all - broodies are extremely protective of their eggs and getting them out and back again can be fraught with difficulty!
If you do candle, please make sure you wash your hands before you do. I can't say this often enough. If you don't, you run the risk of transmitting bacteria to the embryo as the shells become more and more porous during incubation.
What would I see if I candle today and there's no development?
When an egg's not developing you won't see any of the shadows or blood vessels. You will see a darker area which is the yolk, but nothing inside it.
This is an infertile Wyandotte egg candled at day 8 - it's showing no signs of development.
However, don't discard any eggs which seem not to be developing yet, unless they are oozing. Some chicks take longer than others to develop and some, particularly those like Marans which have a very dark shell, are just too hard to see.
The egg in the picture above didn't develop and was discarded at day 14. I never find it an easy thing to do - I always hope against hope - but if left any longer, there's a danger the shell as it becomes more porous will take in bacteria and contaminate all the others.
At each stage of this incubating series I feature a photo of one of my own chicks.
It helps keep our minds focused on what we're working towards - a healthy, happy chicken.
This is one of my Light Sussex chicks, two days after hatching. You can still just see the egg tooth - the small lighter coloured tip at the end of the beak, which the chick uses to break its way through the membrane and out of the shell.
Have you joined my incubating and hatching group yet?
If you're having a go at hatching 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.