Eggshell
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An eggshell is the outer covering of a hard-shelled egg and of some forms of eggs with soft outer coats. Bird eggshells contain calcium carbonate and dissolve in various acids, including the vinegar used in cooking. While dissolving the calcium carbonate in an egg shell reacts with the acid to form carbon dioxide. [1]
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Insect eggs
i ♥ candace 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 embryonic membrane, the choroid, which serves to protect inner layers. The choroid 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. These type of eggs can also be very small and fragile.
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.
Bird eggs
The bird egg is a fertilized (or, in the case of some birds [such as chickens] possibly unfertilized) gamete located on the yolk surface and surrounded by albumen, or egg white. The albumen in turn is surrounded by two shell membranes (inner and outer membranes) and then the eggshell. The chicken eggshell is 95-97% calcium carbonate crystals, which are stabilized by a protein matrix.[2][3][4] Without the protein, the crystal structure would be too brittle to keep its form and the organic matrix is thought to have a role in deposition of calcium during the mineralization process.[5][6][7] The structure and composition of the avian eggshell serves to protect the egg against damage and microbial contamination, prevention of desiccation, regulation of gas and water exchange for the growing embryo, and provides calcium for embryogenesis. Eggshell formation requires gram amounts of calcium being deposited within hours, which must be supplied via the hen’s diet.[4]
The fibrous chicken shell membranes are added in the proximal(white) isthmus of the oviduct.[4] In the distal (red) isthmus mammillae or mammillary knobs are deposited on the surface of the outer membrane in a regular array pattern.[8][9] The mammillae are proteoglycan-rich and are thought to control calcification. In the shell gland (similar to a mammalian uterus), mineralization starts at the mammillae. The shell gland fluid contains very high levels of calcium and hydrogen carbonate. The thick calcified layer ofi destiny
bbbbbbbbbbbbbbbilar 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.
Egg teeth
Hatching birds, amphibian and egg-laying reptiles have an egg-tooth used to start an exit hole in the hard eggshell.<ref>http://ag.ansc.pu
See also
- Eggshell skull rule, in tort law
- Walk on eggshells, an idiom in the English language
References
- ^ http://van.physics.illinois.edu/qa/listing.php?id=461
- ^ Arias, J. L.; Fernandez, M. S. (2001). "Role of extracellular matrix molecules in shell formation and structure". World's Poultry Science Journal. 57: 349–357. doi:10.1079/WPS20010024.
- ^ Nys, Yves; Gautron, Joël; Garcia-Ruiz, Juan M.; Hincke, Maxwell T. (2004). "Avian eggshell mineralization: biochemical and functional characterization of matrix proteins". Comptes Rendus Palevol. 3: 549–62. doi:10.1016/j.crpv.2004.08.002.
- ^ a b c Hunton, P (2005). "Research on eggshell structure and quality: an historical overview". Revista Brasileira de Ciência Avícola. 7: 67–71. doi:10.1590/S1516-635X2005000200001.
- ^ Romanoff, A.L., A.J. Romanoff (1949) The avian egg. New York, Wiley.
- ^ Burley, R.W., D.V. Vadehra (1989) The Avian Egg: Chemistry and Biology. New York, Wiley.
- ^ Lavelin, I; Meiri, N; Pines, M (2000). "New insight in eggshell formation". Poultry science. 79 (7): 1014–7. PMID 10901204.
- ^ Wyburn, GM; Johnston, HS; Draper, MH; Davidson, MF (1973). "The ultrastructure of the shell forming region of the oviduct and the development of the shell of Gallus domesticus". Quarterly journal of experimental physiology and cognate medical sciences. 58 (2): 143–51. PMID 4487964.
- ^ Fernandez, MS; Araya, M; Arias, JL (1997). "Eggshells are shaped by a precise spatio-temporal arrangement of sequentially deposited macromolecules". Matrix biology. 16 (1): 13–20. PMID 9181550.
Further reading
- Kilner, R. M. (2006). "The evolution of egg colour and patterning in birds". Biological Reviews. 81 (3): 383–406. doi:10.1017/S1464793106007044. PMID 16740199.