Eggshell 2781408 215275473 2008-05-27T14:00:54Z Pinkgothic 185141 /* Bird eggs */ Fixed a bit of formatting gone wrong. {{sources|date=February 2008}} [[Image:White chicken egg square.jpg|thumb|250px|The intact white shell of a chicken egg.]] The term '''eggshell''' is a term for the outer covering of a hard-shelled [[egg (biology)|egg]], and some forms of eggs with soft outer coats. The generalized eggshell structure, which varies widely among species, is a [[protein]] matrix lined with mineral crystals, usually of a [[calcium]] compound such as [[calcium carbonate]]. It is calcium build-up and is not made of cells. Harder eggs are more mineralized than softer eggs. == Diversity == === Insect eggs === [[Insect]]s and other [[arthropod]]s lay a variety of styles and shapes of eggs. Some have gelatinous or skin-like coverings, others have hard eggshells. Softer shells are mostly protein. It may be fibrous or quite liquid. Some arthropod eggs do not actually have shells, rather, their outer covering is actually the outermost extraembryonic membrane, the '''chorion''', which serves to protect inner layers. The chorion itself can be a complex structure, and it may have different layers within it. It may have an outermost layer called an ''exochorion''. Eggs which must survive in dry conditions usually have hard eggshells, made mostly of dehydrated or mineralized proteins with pore systems to allow [[respiration (physiology)|respiration]]. Arthropod eggs can have extensive ornamentation on their outer surfaces. === Fish, amphibian and reptile eggs === [[Fish]] and [[amphibian]]s generally lay eggs which are surrounded by the extraembryonic membranes but do not develop a shell, hard or soft, around these membranes. Some fish and amphibian eggs have thick, leathery coats, especially if they must withstand physical force or desiccation. While many [[reptile]]s lay eggs with flexible, calcified, eggshells, there are some that lay hard eggs. Eggs laid by snakes generally have leathery shells which often adhere to one another. Depending on the species, turtles and tortoises lay hard or soft eggs. Several species lay eggs which are nearly indistinguishable from bird eggs. Hatching reptiles, like most birds, have [[egg tooth|egg-teeth]] with which they slit or peck an opening into the shell to help them emerge. === Bird eggs === [[Bird]]s are known for their hard-shelled eggs. The eggshell is approximately 95% [[calcium carbonate]] crystals, which are stabilized by a protein matrix. Without the protein, the crystal structure would be too brittle to keep its form. The standard bird eggshell is a [[semipermeable membrane]] filled with thousands of pores and covered on its outer surface with a '''[[cuticle]]''' (called the ''bloom'' if it is around a chicken egg), which helps the egg retain its water and keep out [[bacteria]]. As an egg develops within a bird's reproductive system, the shell is laid down as a liquid mineral layer around the already-formed extraembryonic membranes. This takes place in a part of the bird's [[oviduct]] which is called the uterus and serves a very different purpose from the uterus of a [[mammal]]. The shell is laid down by '''shell glands'''. In an average [[chicken|laying hen]], the process of shell formation takes around 20 hours. [[Pigment]]ation is added to the shell, coloring it any of a variety of colors and patterns depending on species. The waxy cuticle is deposited on the shell in the vagina of the bird's oviduct. The chick has an [[egg tooth|egg-tooth]] which it uses to start a hole in the hard eggshell to allow it to hatch. === Mammal eggs === [[Monotreme]]s, egg-laying mammals, lay soft-shelled eggs similar to those of reptiles. The shell is deposited on the egg in layers within the uterus. The egg can take up fluids and grow in size during this process, and the final, most rigid layer is not added until the egg is full-size. ==See also== * The [[Eggshell skull]] rule, in [[tort]] law. [[Category:Zoology]] [[Category:Reproductive system]] [[Category:Eggs]] [[es:Cáscara de huevo]]