An egg is a biological structure produced by many animals that contains an embryo or gamete and the nutrients needed for early development. In everyday language, "egg" most often refers to bird eggs, especially chicken eggs, which are widely consumed by humans. In biology, the term also covers the female reproductive cell (ovum) in animals and plants' seed-like structures with analogous functions.
Structure and parts
Typical avian eggs have several distinct layers and components. From outside in they include the hard shell, shells membranes, the albumen (egg white), and the yolk. The shell is mostly calcium carbonate and provides mechanical protection and a semi-permeable barrier for gas exchange. The albumen cushions the embryo and supplies water and protein; the yolk is rich in fats and vitamins and supplies energy for growth. Small structures such as the chalazae help center the yolk, and an air cell forms as the egg cools after laying.
Reproduction and development
Eggs are central to reproduction in oviparous animals (those that lay eggs). Fertilization can be internal (common in birds and reptiles) or external (common in many fish and amphibians). After fertilization the embryo develops inside the egg, drawing on yolk nutrients and exchanging gases through the shell or membranes. Incubation—by parent body heat or artificial devices—maintains the conditions necessary for embryogenesis until hatching.
Uses, cuisine and cultural roles
- Food: eggs are a versatile source of protein, used whole, separated, or processed in baking, sauces, and confections.
- Industrial and scientific: eggs are used in vaccine production, laboratory research, and as a medium in some manufacturing processes.
- Cultural: eggs appear in art, religious symbolism, and festivals (for example, painted Easter eggs) and have been used in traditional crafts like egg tempera painting.
Types, safety and notable distinctions
Eggs vary widely by species in size, color, and shell thickness. Chicken eggs are most common in commerce and may be labeled by production methods (free-range, organic, cage-free), which concern housing and diet rather than intrinsic egg composition. Food safety issues include contamination by pathogens such as Salmonella; cooking, refrigeration, and pasteurization reduce risk. Culinary and biological distinctions include fertilized versus unfertilized eggs (both edible), double-yolk eggs, and the distinction between the ovum as a single cell and the multicompartment egg of birds and reptiles.
Beyond nutrition, eggs are important in ecology and agriculture: they are a key life stage for many species and a cornerstone of animal husbandry, global diets, and economic activity.