Eggs are organic vessels containing a zygote in which an animal embryo develops until it can survive on its own; at which point the animal hatches. 

An egg results from fertilization of an ovum. Most arthropods, vertebrates, and mollusks lay eggs, although some, such as scorpions and most mammals, do not.


A wide variety of animal eggs illustrated. image: Adolphe Millot/ wikipedia

Reptile eggs, bird eggs, and monotreme eggs are laid out of water, and are surrounded by a protective shell, either flexible or inflexible. Eggs laid on land or in nests are usually kept within a warm and favorable temperature range while the embryo grows. When the embryo is adequately developed it hatches, i.e. breaks out of the egg's shell. Some embryos have a temporary egg tooth they use to crack, pip, or break the eggshell or covering.

Parts of a typical shelled egg. image: KDS4444/ wikipedia

Eggs vary a lot in size

The largest recorded egg is from a whale shark, and was 30 cm × 14 cm × 9 cm  in size. Whale shark eggs typically hatch within the mother. 


The egg case of a whale shark. image: 

At 1.5 kg  and up to 17.8 cm × 14 cm, the ostrich egg is the largest egg of any living bird, though the extinct elephant bird and some dinosaurs laid larger eggs. 

The bee hummingbird produces the smallest known bird egg, which weighs half of a gram . Eggs laid by reptiles and most fish, amphibians, insects and other invertebrates can be even smaller.


Magnified frogspawn are aquatic eggs with limited protection. image: Tarquin /wikipedia

Reproductive structures similar to the egg in other kingdoms are termed "spores," or in spermatophytes "seeds," or in gametophytes "egg cells”.

See also: which came first chicken or the egg?

reference adapted from: Egg. (2018, January 26). In Wikipedia, The Free Encyclopedia. Retrieved 22:31, January 26, 2018, from