The Anatomy of a Chicken Egg


I’ve been poking around the internet and books for some cool experiments and information about eggs in general. I came across this really nice breakdown of the various parts of a chicken egg over at and have duplicated the information below. I’ve always wondered what those white stringy things (Chalaza) are in my eggs when I make an omelet and now I know not only their name, but also what their function is inside the eggs!

  1. Eggshell – The outer eggshell is made almost entirely of calcium carbonate (CaCO3) and is covered with as many as 17,000 tiny pores. It is a semipermeable membrane, which allows air and moisture to pass through its pores. The shell also has a thin outermost coating called the bloom or cuticle that helps keep out bacteria and dust (see below 15).
  2. Outer shell membrane
  3. Inner shell membrane – These two membranes – outer and inner – are just inside the shell surrounding the albumen (white). The two membranes provide an efficient defense against bacterial invasion and are made partly of keratin. The outer membrane sticks to the egg shell while the inner membrane sticks to the albumen. When an egg is first laid, it is warm. As it cools, the contents contract and the inner shell membrane separates from the outer shell membrane to form the air cell (see 14 below).
  4. Chalaza – are twisted in opposite directions and serve to keep the yolk centered. The more prominent the chalazae, the fresher the egg.
  5. Exterior albumen (outer thin albumen) — The outer thin albumen is a narrow fluid layer next to the shell membrane.
  6. Middle albumen (inner thick albumen) -The inner thick white (chalaziferous layer) is a dense, matted, fibrous capsule of albumen around the vitelline membrane of the yolk. The matted fibrous capsule terminates on each end in the chalazae, which are twisted in opposite directions and serve to keep the yolk centered. This part of the egg is a excellent source of riboflavin and protein. In high-quality eggs, the inner thick albumen stands higher and spreads less than thin white. In low-quality eggs, it appears thin white.
  7. Vitelline membrane – The clear casing that encloses the egg yolk. When an egg is said to be “mottled”, the yolk surface is covered with many pale spots or blotches. The strength and integrity of the vitelline membrane are very important in preventing egg yolk mottling.
  8. Nucleus of pander – a plug of whitish yolk, with no particular significance for development and whose function is purely a nutritive one, like the rest of the yolk. (See: Int. Schmitt S., (2005) J. Dev. Biol. 49: 1-8).
  9. Germinal disk (blastoderm) – a small, circular, white spot (2-3 mm across) on the surface of the yolk; it is where the sperm enters the egg. The nucleus of the egg is in the blastodisc. The embryo develops from this disk, and gradually sends blood vessels into the yolk to use it for nutrition as the embryo develops.
  10. Yellow yolk – a major source of vitamins, minerals, almost half of the protein, and all of the fat and cholesterol. The yolk contains less water and more protein than the white, some fat, and most of the vitamins and minerals of the egg. These include iron, vitamin A, vitamin D, phosphorus, calcium, thiamine, and riboflavin. The yolk is also a source of lecithin, an effective emulsifier. Yolk color ranges from just a hint of yellow to a magnificent deep orange, according to the feed and breed of the hen.
  11. White yolk – Also known as, the latebra is an area of white yolk located in the center of the yolk. It is lower in fat and therefore stands out as a bright white area in many Magnetic Resonance Images. The specific function of the latebra is uncertain but it may act as a central structure around which the additional layers of the yolk are formed.
  12. Internal albumen (Chalaziferous albumen)– The inner thick white (chalaziferous layer) is a dense, matted, fibrous capsule of albumen around the vitelline membrane of the yolk. The matted fibrous capsule terminates on each end in the chalazae, which are twisted in opposite directions and serve to keep the yolk centered.
  13. Chalaza chalazae, which are twisted in opposite directions and serve to keep the yolk centered. The more prominent the chalazae, the fresher the egg. chalazae, which are twisted in opposite directions and serve to keep the yolk centered.
  14. Air cell – An air space forms when the contents of the egg cool and contract after the egg is laid. The air cell usually rests between the outer and inner membranes at the eggs larger end. As the egg ages, moisture and carbon dioxide leave through the pores of the shell, air enters to replace them and the air cell becomes larger.
  15. Cuticle or bloom – The shell is produced by the shell gland (uterus) of the oviduct, and has an outer coating, the bloom or cuticle. The cuticle somewhat seals the pores and is useful in reducing moisture losses and in preventing bacterial penetration of the egg shell. Most of cuticle is removed from table eggs when they are mechanically washed.

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About the author

Carl Nelson is the Chief Scientist at Imagination Station in Toledo, Ohio. He holds a Masters Degree in Experimental Physics from Michigan State University and has been having fun exploring (exploding?) science in Science Centers for the past 20 years.


41 Responses to “The Anatomy of a Chicken Egg”
  1. Katherin Sniffin says:

    Great Egg info! I can use this at different levels with students K-8 Thanks!

  2. Trevor Morgan says:

    I love this site because I’m doing a science paper on the egg so I need all the information I can get.

  3. Azhar Rajah says:

    Eggcellent information

  4. Judi Davis says:

    First year with laying hens. So, naturally, I loved your information of the egg. Now I will be quite inforative as I answer questions from my grandchildren. But here is my question: occasionally, one of my hens lays a egg with outershell covered with a granular hard substance. If I pick these little hard specks, the shell opens. or leaves a hole where the speck was.
    What are these? I thought maybe, I am giving her too much vitamin C in her ‘snacks’ (apples, grapes etc) I need to know, as I might explain to the family. :) Thank You Judi Davis

  5. Carl says:

    Hi Judi, I’m not sure what those specks may be.

  6. Samuel says:

    Sir, what is the protine in yolk?

  7. Carl says:

    Egg white proteins are the principal solutes present in egg white, making up approximately 10% of its weight. Ovalbumin, ovotransferrin and ovomucoid are the proteins contained in the white and make up approximately 75% of the total protein in this part of the egg.

    From the USDA Nutrient Database the following amino acids, which make up proteins, are found in the yolk of an uncooked chicken egg.
    Yolk Protein 15.86 g
    – Tryptophan 0.177 g
    – Threonine 0.687 g
    – Isoleucine 0.866 g
    – Leucine 1.399 g
    – Lysine 1.217 g
    – Methionine 0.378 g
    – Cystine 0.264 g
    – Phenylalanine 0.681 g
    – Tyrosine 0.678 g
    – Valine 0.949 g
    – Arginine 1.099 g
    – Histidine 0.416 g
    – Alanine 0.836 g
    – Aspartic acid 1.550 g
    – Glutamic acid 0.595 g
    – Glycine 0.488 g
    – Proline 0.545 g
    – Serine 1.326 g

  8. anna says:

    Who came up with all thes names for the egg layers?

  9. Geno says:

    Very helpfull, It’s on my test for being a chef.

  10. theresa j. cervantes says:

    I thought that the yolk was the chick and the white was it’s food. I found out that the opposite is this true?

  11. Nina Arciaga says:

    why is it easier for a chicken inside an eggshell to poke its way out than it is for a chicken on the outside to poke its way in?

  12. Dee says:

    The yolk is full of cholesterol in an egg. What happens to that cholesterol when the egg is allowed to develop into a chicken? Chicken meat is not high in cholesterol.

  13. Carl says:

    Hi Bijouxici,

    Great question! I have to admit that I don’t know the answer at this time. I will search for more info about this question and post an update on the site.


  14. Dakota Volk says:

    Thank you for publishing this article it was just what I was looking for my science project.great details were shown.My Grandma told me the egg is a complete protein and the lecithin is a emulsifier which binds with fat and removes it from our bodies systems when eaten.

  15. Christine says:

    This stuff is amazing! What got you interested in it in the first place?

  16. Viridiana Maya Villasenor says:

    i still dont get what the egg is made out of… sorry, :( could u explain further?

  17. Carl says:

    Hi Christine,

    I first did this experiment as a 5th grader and have found it interesting ever since.


  18. Carl says:

    Hi Nina,

    Well, a chicken on the outside will apply a force to the shell that is distributed into the shell much like the force applied to an arch bridge. Each part of the egg below the force will support the part of the shell above it.

    When the chick inside pokes outward on the shell, there is no material outside to support that force. The shell will break open easily.

    Hope that helps,

  19. jim malone says:

    If a chicken lays a double yolk wil there be twin chicks?

  20. Carl says:


    Good question, and I don’t have a good answer for that. Anyone else out there have experience with this?


  21. Shirley says:

    The knobbly bits on the shell are calcium deposits.. Look up egg shell quality.
    I have a question though…. When I boil my eggs some of the shells don’t peel off properly do you know why? Something to do with the me brains I think.

  22. Shirley says:

    To answer the double yoke question… Usually not here isn’t enough room for two chicks to develope they usually die before hatching.

  23. Dayeon Lee says:

    I wanna eat eggs!

  24. saj says:

    Ya two yolk will give birth to two chickens
    we know that nucleus is present in blastopor of egg
    thats why two yolk will have two blastopores and nuclus
    so two nucleus of eggs will be fertalized…to give bith to chks

  25. Lauren Robinson says:

    Is there anymore info on air cells??

  26. Maggie says:

    What causes no membrane around the yolk? Are those eggs safe to eat?

  27. huda says:

    in ovo injection ? whats that meaning ,what useful from making hole in egg ??

  28. Freddie Abuda Jr. says:


  29. Carl says:

    “In ovo” is Latin for “in the egg.” In refers to the growth of live virus in chicken egg embryos for vaccine development for human use.

  30. hero vet says:

    why layers of egg yolk are light yellow and dark yellow????????

  31. Jayffle says:

    All these egg puns are terrible, to be honest. All I want is a decent eggucation

  32. Carl says:


  33. Judy Pena says:

    what does it mean, if the egg does not have that chalaza? Does it mean something is wrong with it ? Is it old or is the egg just as fresh without it? How can you use the eggshell membrane for knee joint relief?

  34. danny mcegg says:

    we compared eggs to plant cells. eggsuberant data on the egg

  35. Jakari says:

    Very helpful! Great information to put on my essay! =)


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