The term eggshell is a term for the outer covering of a hard-shelled 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.


Insect eggs

Insects and other arthropods 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. Arthropod eggs can have extensive ornamentation on their outer surfaces.

Fish, amphibian and reptile eggs

Fish and amphibians 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 reptiles 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-teeth with which they slit or peck an opening into the shell to help them emerge.

Bird eggs

Birds 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 laying hen, the process of shell formation takes around 20 hours. Pigmentation 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 which it uses to start a hole in the hard eggshell to allow it to hatch.These eggs, since they are composed mainly of Calcium Carbonate, have excellent abrasive properties when crushed, depending on the coarseness of the crushed shell.

Mammal eggs

Monotremes, 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.

ee also

* The Eggshell skull rule, in tort law.

Wikimedia Foundation. 2010.

Поможем решить контрольную работу

Look at other dictionaries:

  • eggshell — egg shell , a. 1. of a pale, yellowish white color; as, an eggshell ceiling and light green walls. [1913 Webster +PJC] 2. (Architecture) having a smooth but not glossy texture like that of a hen s egg; as, a latex paint giving an eggshell finish …   The Collaborative International Dictionary of English

  • eggshell — egg shell , n. 1. The shell or exterior covering of an egg. Also used figuratively for anything resembling an eggshell. [1913 Webster] 2. (Zo[ o]l.) A smooth, white, marine, gastropod shell of the genus {Ovulum}, resembling an egg in form. [1913… …   The Collaborative International Dictionary of English

  • eggshell — [eg′shel΄] n. the hard, brittle outer covering of an egg of a reptile, bird, or monotreme adj. 1. fragile and thin, like eggshell 2. yellowish white …   English World dictionary

  • eggshell — early 15c., from EGG (Cf. egg) (n.) + SHELL (Cf. shell). Earlier ay schelle (c.1300). Emblematic of thin and delicate from 1835; as a color term, from 1894. The figure of treading on eggshells to move cautiously is attested by 1734 …   Etymology dictionary

  • eggshell — ► NOUN 1) the thin, hard, fragile outer layer of an egg. 2) (before another noun ) (of china) extremely thin and delicate. 3) (before another noun ) denoting an oil based paint that dries with a slight sheen …   English terms dictionary

  • eggshell — I UK [ˈeɡˌʃel] / US noun Word forms eggshell : singular eggshell plural eggshells 1) [countable] the hard outside layer of an egg 2) [uncountable] a type of paint that is slightly shiny II UK [ˈeɡˌʃel] / US adjective like an eggshell, for example …   English dictionary

  • eggshell — 1. noun a) The shell around an egg. b) A pale off white colour, like that of the eggshell. 2. adjective a) Of a pale yellowish whitish colour, like that of the eggshell. b) …   Wiktionary

  • eggshell — egg|shell1 [ eg,ʃel ] noun 1. ) count the hard outside layer of an egg 2. ) uncount MAINLY BRITISH a type of paint that is only slightly shiny eggshell egg|shell 2 [ eg,ʃel ] adjective like an eggshell, for example white yellow in color, not very …   Usage of the words and phrases in modern English

  • eggshell — [[t]e̱gʃel[/t]] eggshells (The spelling egg shell is also used for meaning 1.) 1) N VAR An eggshell is the hard covering on the outside of an egg. 2) ADJ: usu ADJ n Eggshell paint makes surfaces dull, not shiny, when it is painted on them. The… …   English dictionary

  • eggshell — /eg shel /, n. 1. the shell of a bird s egg, consisting of keratin fibers and calcite crystals. 2. a pale yellowish white color. 3. rather bulky paper having a slightly rough finish. adj. 4. like an eggshell, as in thinness and delicacy; very… …   Universalium

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”