Here is a list of both achievement-based and secret eggs listed in order of appearance in the game and a little guide on how to obtain each of them:
Egg 1: Chapter 1
Achievement: Yes! - Wrong Way
Location: At the very start of the game when the character wakes up, run to the left instead of the right and you should find the egg sitting on the ground.
Egg 2: Chapter 2
Achievement: Yes! - Altitude is Attitude
Location: At the point where you need to deal with the wooden box floating in the water and you climb the trees. From the tree branch you drop the broken logfrom, you can run and jump to the left to grab a hard to see rope. Use this to swing up to a higher platform on the left, here you can pick up the egg.
Egg 3: Chapter 8
Location: In the area with the mine cart and the rope suspended above the two spiked cogs. Climb the rope here and once the rope stops coming down, jump backto the ladder on the left. Climb up and run to the left into the previous chapter. Drop down into the first hole you come across for the egg. Quickly jump out again unless you want to be impaled by spikes.
Egg 4: Chapter 9
Location: Load up chapter 9, from the start point immediately run to the left. Grab the crate and push it as far as you can to the left and position it so it is sitting on the edge of the ledge. Return to the right, scare the creature into the hamster wheel and get the rain to start. Once this has been done, run back to the crate, there should be an egg on top of it.
Egg 5: Chapter 11
Achievement: Yes! – Its Stuck
Location: Once you have ditched the brain slug in the room with the rising water, climb back down the ladder and pull the floating pipe you used earlier to jump across. Push the pipe out as far as you can to the right so that its left side is only just touching the platform. Jump on it, and then do a running jump back to the left. You will grab onto a rope partially hidden by the environment. As you pull on this, an egg will fall out of the pipe above and land on the floating pipe.
Egg 6: Chapter 14
Achievement: Yes! – Urban Exploration
Location: In the room with the lift. Move the crate up to the second level. Hit the switch to lower the platform. Hit the switch to send the lift up again, but use the crate to jump on top of the lift. Ride the lift up and as you near the top, look to the left, you should see a rope hanging off to the side. Jump over to this and an egg will pop out of a pipe higher up on the wall. Jump backto the lift and drop down to the ground level. Cruise to the right to pick up the egg.
Eggs 7: Chapter 14
Achievement: Yes! – Alone in the Dark
Location: Load up chapter 15 and climb down the ladder to the previous area. Below the ladder you will come across a dark cave. Enter the cave and follow it downwards. You will be unable to see anything. But when you hear the character step in water, jump across to the right. If successful, he will grab a platform and climb up. Run to the right to find the egg.
Eggs 8 – 10: Chapter 14
Location: From the location of the above egg (for the achievement Alone in the Dark), run to the right until you hit a wall. Hold the action button and hold back on the stick, you will pull out a block. Jump onto this and onwards. Continue to the right to find egg 8.
Just after egg 8, listen to your characters footsteps, he will stand on something that sounds a little different. Continue to the right until you hit another wall. Jump up here, then turn around and perform a running jump to the left, he should land on the strange sounding ground and fall through to an area below. Follow the passageway down to the right to find egg 9.
From here continue right until you hit a wall again, this time press down onthe control stick and drop down to a platform below. Run to the right and jumpup when you reach a wall. Continue to the right and you will see egg 10 below you. Listen for your footsteps, when you hear clanging drop off the platform tothe right and hit the use button, pull the box out of the way, then jump down to grab the egg.
Egg 11: Chapter 16
Achievement: Yes! – Climbing the Cog
Location: Once you have the box and have reached the first large cog, jump upto the platform with the lever. Instead of pulling the lever, use the platform to jump to the large cog to the right of your current location. You should be able to climb it. Make your way over to the opposite side of the cog and you should see the egg.
Egg 12: Chapter 17
Location: After starting the room rotating for the first time and you climb thefirst cog, continue to climb it and head all the way over the top of it and to the right. In the darkness above the cog you will find the egg.
Egg 13: Chapter 18
Achivement: Yes! - Backtracking
Location: After ditching the brain slug on the conveyors, stand on the left hand side of the third conveyor and wait for the crusher to descend, use one ofthe crates to jump up and grab onto the right hand ledge of the crusher. Ride it to the top and when it is safe to do so, jump down to the left. When the crusher comes down again, hang onto the left side of it and at the top, jump across to the hanging ladder nearby. Climb to the top and follow the passagewayto the right to find an egg, squish it for the achievement.
Egg 14: Chapter 19
Achievement: Yes! - Guided by Sparks
Location: After the level rotation puzzle in the chapter you will end up on a platform with an electrified wall to the right, and in front of it a hard to spot wooden crate. Push this up against the electric wall and get on top of it.Jump to the left to climb up onto a hidden platform. From here jump to the ledge above the electric wall to find an egg.
Egg 15: Chapter 21
Location: Load up chapter 21 and drop down into the pit. Grab the box furthest to the right and push it over and activate the gravity switch. When The timer expires, quickly drag this same box to the right wall, so that when the other box returns, they have changed places. push the box that is now furthest to theright past the switch and up the hill. Let the box go and run and jump onto thesecond box to reach the starting platform. Once the gravity switch has been activated and the time expires, grab the box as it is sliding down the ramp.Look at it closely and make sure the top of the box has a couple of small protrusions on it (if not, repeat the previous step until the two protrusions are facing upwards). Push the box all the way back to the previous chapter and push it beneath the vertical gun turret. It should blow a hole in the top of the box. Push the box all the way back to where you found it, and an egg will fall out of it.
Egg 16: Chapter 21
Achievement: Yes! - Under Ground
Location: Run up to the elevator and press down. You will climb down the edge. Jump to the chain here and climb all the way to the bottom to find an egg.
Egg 17: Chapter 21
Location: When you reach the room with the lift and the pair of gravity switches, liberate the two boxes and invert gravity so they ar both in the same area. Put gravity back to normal and bring the two boxes to the elevator. Push one box to the ground level to the left of the elevator, and the second box into the elevator. From here, if your timing is ok, you can push the elevator box ontop of the other to have a double stack. Bring the elevatro down to ground level and slowly and carefully load them onto it. Hit the switch and ascend with them. Slowly push the box to the right hand wall of the room and climb the ladder. Hit the gravity switch to bring the double stack to the roof next to you. Push the double stack slowly to the left as far as you can go. Then use them to jump up to a higher platform on the left. Follow this to the end for an egg.
Egg 18: Chapter 23
Location: Load up chapter 23, run to the top of the vertical wall and jump to the gravity switch and land on the H when it is safe to do so. Wait for the saw to go back down again, then jump across from the H to the top of the vertical wall. Wait until you can barely hear the saw at all any more, and then perform arunning jump to the left, as you fall hit the gravity panel to be tossed back to the vertical wall. Return to the top and jump to the H again, this time jump off to the platform to the right of the H and you will see an egg fall down from a pipe.
Egg 19: Chapter 23
Location: Load up chapter 12. Jump across the neon H to the letter O. You should see a line running through the centre of it. At the end of this line is a handle. We cannot use this yet, but we can position it to where we are able to use it late rin the game. Rotate the O by walking slowly near its edges until the handle is pointing about 2/3 of the way down the right hand side of the O. Now play through the game until chapter 23. After getting past the H here, ride the rope down to the O, if you placed it correcly earlier, the handle should be within reaching distance of the solid platform to the right. Pull the handle and the egg will pop out!
Egg 20: Chapter 24
Location: This egg appears after the short cinematic following the final puzzle. It will only appear if you have started a new game, played through the entire game in one sitting and have not died once. When you regain consciousness after the final puzzle, run all the way to the left, then run back to the right, you should see the egg fall from a tree.