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Ohio State University Extension


The Making of an Egg

Veterinary Preventive Medicine
Timothy McDermott DVM, Educator, Agriculture and Natural Resources, Ohio State University Extension, Franklin County
Jenny Lobb RD LD, Educator, Family and Consumer Sciences, Ohio State University Extension, Franklin County

Eggs are a nutritious food that contain essential vitamins, minerals, lipids, and amino acids. In fact, eggs are a source of all nine essential amino acids, making them a complete protein.

In a hen, it takes approximately 24–26 hours to produce an egg. Hens start egg production when they reach sexual maturity at around 20–22 weeks of age. The reproductive tract of a hen has two parts: the ovary and the oviduct. The oviduct is comprised of five sections: the infundibulum, the magnum, the isthmus, the shell gland, and the vagina.

Labeled hen's reproductive tract, including ovary, ovum, isthmus, uterus, vagina, and cloaca. The contents found inside a hen's egg, including the chalazae, thin albumen, thick albumen, and ovum. A hen's egg.

The following descriptions are correlated to the letters identifying the different parts in egg production as displayed in Figure 1:

A. The ovary is a cluster of round yolk containing follicles of various sizes. Egg production is initiated when the largest mature follicle or ovum is released into the infundibulum from the ovary. At this point, the egg can be fertilized, if there are live sperm present, although egg production does not rely upon fertilization. The ovum spends about 15 minutes in the infundibulum.

B. The ovum proceeds next to the magnum where a protective, nutritious layer (albumen or egg white) is added. There are two layers to the albumen, thick and thin, which serve as sources of niacin, riboflavin, potassium, sodium, and other nutrients. The thick layer is added first and is closest to the yolk. It provides the majority of the riboflavin and protein for the embryo. In addition to the albumin layers, the chalazae are added. These rope-like structures, appearing on both sides of the yolk, are designed to hold the yolk in place as it proceeds through the reproductive tract. The yolk spends about three hours in the magnum.

C. The isthmus is the next destination for the egg. Here, two shell membranes are added in a little over an hour, providing additional protection to the egg from the environment.

D. The uterus, also known as the shell gland, is where the developing egg spends the majority of the time, approximately 20–21 hours. A thin albumin of water and minerals passes through the membranes, causing the egg to inflate into its normal shape. In the shell gland, calcium in the form of calcite crystals is then secreted, forming the hard outer shell. Finally, a protective outer coating, called the cuticle, is secreted over the egg before the egg enters the vagina.

E. The vagina is the last part of the reproductive tract the egg passes through. The egg rotates prior to being laid in a process called oviposition so that the large end comes out first.

F. After production is complete, the egg passes through the cloaca and is laid.

An air pocket forms between the two shell membranes and the egg’s outer shell. This pocket starts to cool after the egg is laid and is used by the chick to breathe before it hatches.

Interestingly, if a hen has a red earlobe, the egg will usually be colored brown in this stage of egg production. If the hen’s earlobe is white, the egg is usually white too.

For information on safe handling and washing of eggs please see the Ohioline fact sheet, Selling Eggs in Ohio: Marketing and Regulations, ANR-59.


    Auburn University. 2019. Virtual Chicken: Part 1: The Female Reproductive Tract. Auburn University, Department of Poultry Science. Video.

    Jacob, Jacquie. n.d. “Avian Reproductive System – Female.” Small and Backyard Poultry, United States Cooperative Extension System.

    Réhault-Godbert, Sophie, Nicholas Guyot, and Yves Nys. 2019. “The Golden Egg: Nutritional Value, Bioactivities, and Emerging Benefits for Human Health.” Nutrients Volume 11, Issue 3: 684.

    Wilson, Peter W., Ceara S. Suther, Maureen M., Bain, Wiebke Icken, Anita Jones, Fiona Quinlan-Pluck, Victor Olori, Joël Gautron, and Ian C. Dunn. 2017. “Understanding Avian Egg Cuticle Formation in the Oviduct: A Study of Its Origin and Deposition.” Biology of Reproduction Volume 97, Issue 1: 39–49.

    This fact sheet was originally written by Sara J. Spiegle B.S., Avian Disease Investigation Laboratory; Aaron J. Ison B.S., Avian Disease Investigation Laboratory; and Dr. Teresa Y. Morishita DVM, Ph.D., Dipl. ACVP, Ohio State University Extension-Veterinary Medicine & Avian Disease Investigation Laboratory

    Originally posted Jan 3, 2022.