Biology Dictionary – E

Meanings of biological terminology

EUGENE M. MCCARTHY, PHD GENETICS

Definitions:

e (1) electron; (2) electric charge.

E (1) Escherichia; (2) energy; (3) einsteinium; (4) reduction potential; (5) electromotive force.

ead. Eadem (Latin: the same).

EAHF Medical acronym: eczema, asthma, hay fever.

ear, external The external ear is composed of all parts of the ear outside the eardrum (tympanic membrane).

Ear structure (Enlarge image)

ear, internal The internal ear (or inner ear) is composed of the cochlea, the vestibule, and the semicircular canals (see figure at right).

ear, middle The middle ear is a cavity in the temporal bone. Also called the "typanum cavity," it lies just inside the eardrum (tympanic membrane). Anteriorly, the eustachian tube connects it with the pharynx. There are two membrane-covered openings into the inner ear, and three small bones, or ossicles (the malleus, incus, and stapes), which connect the eardrum with one of these openings, the fenestra vestibuli (the "elliptical window").

earlywood (also springwood) Xylem lignified during the spring. Compare: latewood

eburnation /eh-bur-NAY-shən/ Alteration of a bone to make it dense and ivorylike.

ecaudate /ee-CAWD-ate/ Tailless.

ecbolic /ek-BALL-ik/ Inducing labor or abortion.

ecdysis (pl ecdyses) /EK-də-səs; pl: -seez/ n. Molting or shedding. — ecdysial /ek-DIZ-ee-əl, -DIZH-yəl/

ecdysone /EK-də-sone/ A hormone that causes molting in arthropods.

Echinococcus /eh-KIN-ō-cock-əs/ Dog tapeworm; minute tapeworms infesting dogs, humans, and other carnivores. Also infests non-carnivores as intermediate hosts. Within the host, Echinococcus usually attacks the liver first, and then the lungs. In these organs during the larval stage, fluid-filled brood cysts are formed known as hydatids, which may grow for years reaching enormous sizes. DIAGRAM OF LIFE CYCLE | PICTURE OF HYDATID CYSTS FROM HUMAN LUNG

echinoderm /eh-KINE-ō-derm/ A member of the phylum Echinodermata, marine organisms with internal, calcareous skeletons, five-part radial symmetry, and a water vascular system. Examples are starfishes, sand dollars, crinoids, and sea cucumbers.

echolalia /EK-ō-LAL-ee-yə/ Parrotlike repetition of the vocalization of others.

echolocation /EK-ō-lō-KAY-shən/ A capability of certain organisms, as bats, to locate prey by emitting sounds and listening to the resulting echos.

echopraxia /EK-ō-PRACKS-ee-yə/ Mechanical, pathological repetition of the actions of others.

eclampsia /eh-KLAMP-see-yə/ A pregnancy-associated toxemia. Symptoms: high blood pressure, oliguria, albuminuria. Extreme cases may involve convulsions or coma.

ecoid (Brit. oecoid) /EE-coid/ The skeleton of an erythrocyte.

ecological niche /ee-kə-LODGE-ik-əl, eck-ə-/ The set of conditions, both with respect to the environment and with respect to associations with other organisms, to which a particular type of organism is suited.

ecology /ik-AWL-ə-gee, eek-AWL-/ The study of the interaction of organisms with each other and with their environment — ecologist /ik-AWL-ə-jist, eek-AWL-, -jist, -jəst/

ecosystem /EE-kō-SIS-təm/ The living organisms in a particular place, together with their physical environment, viewed as a functioning system.

ECT Electroconvulsive therapy.

ectoderm /ECK-tō-durm/ Outer cell layer of an embryo. During the course of development it gives rise to the nervous system, the sensory organs, and such superficial structures as the epidermis, hair, pigment cells, and mammary glands. ANIMATION OF HUMAN EMBRYONIC AND FETAL DEVELOPMENT

ectoentad /EHK-tō-EHNT-ad/ Inward from the exterior.

ectopia /ehk-TŌP-ee-yə/ Abnormal placement of an organ or other bodily structure.

ectopic /ehk-TAWP-ik or ehk-TŌP-ik/ Occurring in an abnormal position or manner. Compare: entopic.

ectopic pregnancy /ehk-TAWP-ik or ehk-TŌP-ik/ A life-threatening condition in which the embryo implants outside the uterine cavity.

ectopotomy /ehk-tō-PAWT-ə-mee/ Surgical removal of an ectopic fetus.

ectosarc /EHK-tə-sark/ The outer portion of the protoplasm of certain microorganisms (e.g., amoebas).

ectotherm /EHK-tō-therm/ An animal that uses environmental temperatures and behavior to regulate its temperature (commonly termed "cold-blooded").

ectotoxemia /EHK-tō-tawks-EEM-ee-yə/ Toxemia resulting from a toxin that has been introduced into the body.

eczema /EHK-zim-ə/ An inflammatory skin condition characterized by erythema, itching and oozing, scaling lesions. PICTURE1 | PICTURE2

edema (British: oedema) /ed-EEM-ə/ Excessive fluid in the bodily tissues.

edentate /ee-DEHNT-ayt/ (1) toothless; (2) a member of an order, Edentata, of New World mammals, including sloths, armadillos, and tamanduas (among these, only tamanduas are toothless); (3) being or pertaining to an edentate.

edentia /ee-DEHN-shee-yə/ Lack of teeth.

edentulous /ee-DEHN-chə-ləs/ Toothless. Compare: dentulous.

EEG Electroencephalogram.

Eemian /EEM-ee-ən/ (also known as the Sangamonian Stage in North America or the Ipswichian Stage in the UK) The most recent interglacial of the Ice Age (the current period, the Holocene, in which the climate has been similar to that of the Eemian, may also turn out one day to be an interglacial). The Eemian began about 130,000 years ago and ended some 114,000 years ago.

EF Elongation factor; elongation factors are proteins facilitating translational elongation.

efferent /EF-er-rənt/ Leading or carrying away.

egest /ee-JEST, ə-JEST/ Broadly: to pass waste matter out by any means; specifically: to pass undigested matter out of the digestive tract.

egesta /ee-JEST-ə/ Egested matter.

egestion /ee-JES(H)-chən/ The process of egesting.

EGF Epidermal growth factor, a molecule important in the regulation of cell growth, proliferation, and differentiation. It acts by binding to the epidermal growth factor receptor (EGFR).

egg (1) female sex cell or gamete (ovum); (2) externally laid, fertilized ova.

eiloid /EYE-loid/ Coil-like.

ejaculation /ə-JACK-yə-lay-shən, ee-JACK-/ Discharge of fluid from a duct, in particular from the male urethra, or vaginal glands.

elastin /ə-LAST-in or ee-LAST-in/ A protein that is the main component of yellow elastic tissue.

elbow /EL-bō/ The joint between the human upper arm and forearm; the term is also applied to the equivalent joint in certain animals other than human beings (e.g., dogs, apes). ELBOW ANATOMY

elastinase /ə-LAST-in-aze/ An enzyme that breaks down elastin.

electrolysis /ə-leck-TRAWL-ə-sis/ Production of a chemical change by means of the application of an electrical current.

electrolyte /ə-LECK-trə-lite/ (1) a solution that conducts electricity; (2) any substance that conducts electricity when in solution.

electronegativity /ə-LECK-trə-NEG-ə-TIV-ə-tee/ Ability of an atom to attract electrons to itself.

electrophoresis /ə-leck-trə-for-EE-səs/ A method of separating large molecules of different sizes and/or electrical charges. Typically the molecules are allowed to migrate under the influence of an electric current through an acrylamide or agarose gel. The molecules in question are usually proteins or nucleic acid fragments. The rate of migration depends on the different molecules' sizes and electrical charges so that distinct classes of molecules become separated as they migrate.

electroporation /ə-leck-trə-pore-AY-shən/ A process that uses high-voltage current to introduce DNA into cells through cell membranes that would otherwise be impermeable. See also: transfection.

element /EL-ə-mənt/ A substance that cannot be separated into substances other than itself by chemical means. PERIODIC TABLE

Elephas /EL-ə-fəs/ The genus to which the Asiatic elephant is assigned. DIET OF THE ASIATIC ELEPHANT

elongation /ə-long-GAY-shən, ee-long-/ (1) the construction of a protein by the successive addition of individual amino acids; (2) the lengthening of any chainlike molecule by means of the successive addition of individual units.

elytritis /el-ə-TRITE-əs/ Vaginal inflammation.

elytroptosis /EL-ə-trawp-TŌ-səs/ Vaginal prolapse.

elytrostenosis /EL-ə-trō-stə-NŌ-səs/ Narrowing of the vagina.

embolism Obstruction of a blood vessel.

embryo /EM-bree-ō/ The earliest of stage of development of a plant or animal. In humans, embryo is used up to the third month of pregnancy. Thereafter, once the basic body shape has formed, the term fetus is employed. PICTURE OF CHICK EMBRYO

embryogeny or embryogenesis /EM-bree-AW-jen-ee, EM-bree-ō-JEN-ə-sis/ The process of embryo production.

embryology /em-bree-AWL-ə-jee/ The study of embryos.

embryonic /em-bree-AWN- əck. -ick/ Of or pertaining to an embryo.

embryonic tissue Undifferentiated tissue with the potential to develop into any of the various specialized tissues.

embryonic stem (ES) cells An embryonic cell that can continue to replicate indefinitely and serve as a source of cells that can change into other, specialized kinds of cells.

embryo sac /EM-bree-ō/ A structure composed of eight haploid cells found in angiosperms. It forms by division of a megaspore and represents the female gametophyte stage of the plant's life cycle.

encephalitis /en-SEF-ə-LĪ-təs/ Inflammation of the brain.

encyst /en-SIST/ To form a cyst.

endemic /en-DEH-mick/ (1) adj. native to a given geographic region (or being such a region); (2) n. an organism or disease normally occurring within a given geographic region. Compare: exotic

endergonic /end-er-GAWN-ick/ A nonspontaneous chemical reaction that absorbs free energy. Compare: exergonic

endocrine glands /END-ə-krən, -krin/ Glands secreting into the bloodstream or lymphatic system (and not onto an epithelial surface).

endocrinology /end-ə-krən-AWL-ə-jee/ The study of the endocrine glands.

endocytosis /end-ə-site-TŌ-səs/ A process in which a particle penetrates the plasma membrane of a eukaryotic cell, during which a portion of the membrane invaginates the particle and then pinches off so that the invaginated particle becomes a free vesicle within the cytosol. Compare: exocytosis

endoderm /END-ə-derm/ (also entoderm /ENT-ə-derm/) The inner layer of cells of an embryo. During the course of development it gives rise to the digestive system and its associated glands, including the liver, the respiratory organs, the urethra, urinary bladder, and the vagina.

endogenous virus /ehn-DAWJ-ə-nəs/ A virus inherited from a parent (because copies of it were present in the parent's genome).

endometrium /END-ō-MEET-tree-əm/ The mucous membrane lining the uterus.

endonuclease /END-ə-N(Y)OOK-lee-ayz/ See: restriction endonucleases.

endoplasmic reticulum (ER) /END-ə-PLAZ-mik/ TA complex organelle present in eukaryotic cells. MORE INFORMATION

endosarc /END-ə-sark/ The inner portion of the protoplasm of certain microorganisms (e.g., amoebas).

endoskeleton /END-ō-SKEL-ə-tən/ An internal skeleton, as opposed to an exoskeleton.

endosome /END-ə-sōm/ (1) a vesicle produced by endocytosis; (2) a conspicuous particle, other than chromatin, within the nucleus of a protozoan.

endosperm /END-ə-sperm/ Triploid cells that surround and nourish the embryos of seed plants.

endostosis (pl endostoses) /END-aw-STŌ-səs; pl: -seez/ Formation of a tumor within a bone.

endosymbiont theory /end-ō-SYM-bī-awnt/ A theory, now largely accepted, that the chloroplasts and mitochondria of eukaryotic cells are the descendants of free-living prokaryotes.

endosymbiosis /end-ō-sym-bī-Ō-səs/ A special form of symbiotic association between cells, in which one type of the cell lives inside the another. See: endosymbiont theory.

endosymbiotic /END-ō-SYM-bī-AWT-ick/ Characterized by endosymbiosis.

endothelial /END-ə-THEEL-ee-əl/ Of, pertaining to, or being endothelium.

endothelium /END-ə-THEEL-ee-əm/ The layer of cells lining an internal body cavity.

endotherm /END-ə-therm/ An animal that uses metabolic heat to regulate its temperature (commonly termed "warm-blooded").

energy /EN-er-jee/ The capacity or potential to do work. Living organisms store energy in molecules, such as carbohydrates and lipids. In many biological processes, the energy released from such molecules is carried and transported by the ATP.

enology (British: oenology) /ee-NAWL-ə-jee/ The scientific study of wine and winemaking.

enteritis /ehn-ter-Ī-tis/ Inflammation of the intestines, especially of the mucous membranes of the small intestine.

enterology /ehn-ter-AWL-ə-jee/ The study of the intestinal tract.

enteropathy /ehn-ter-AWP-ə-thee/ Any disease of the intestines.

enterorrhaphy /ehn-ter-ORE-ə-fee/ Stitching of an intestinal wound or of the intestines to some other structure.

enterorrhexis /ehn-ter-ō-REX-əs/ Rupture of the intestines.

entoderm See: endoderm.

entomology /ent-ə-MAWL-ə-jee/ The study of insects — entomologist /ent-əm-AWL-ə-jist/

entomophagous /ent-ə-MAFF-ə-gəs/ Feeding on insects.

entopic /en-TAWP-ick/ Occurring in the normal location. Compare: ectopic.

entoptic /ent-AWP-tick/ Lying or occurring within the eye.

entoretina /ENT-ō-REHT-ə-nə/ The portion of the retina composed of nervous tissue.

enuresis /en-yer-EE-sis/ Habitual discharge of urine during sleep.

enzyme /EN-zīm/ A protein catalyst.

Eocene /EE-ō-seen/ An epoch of the Paleogene of the Cenozoic Era. It lasted from 55.8 ± 0.2 to 33.9 ± 0.1 mya. GEOLOGIC TIME SCALE

eosin /EE-ə-sin/ An acid stain that acts as a fluorescent red dye, much used in microscopy; stains collagen, cytoplasm, and muscle fibers; most often used as a counterstain to haematoxylin in haematoxylin and eosin (H&E) staining, a primary technique in histology.

eosinophil (also eosinophile) /ee-ə-SIN-ə-fil, ee-ə-SIN-ə-file/ A leukocyte that stains red with eosin. Eosinophils target parasites and play a role in allergic reactions.

ependyma /ep-END-ə-mə/ The epithelial lining of the spinal canal and brain cavity.

ependymal cell /ep-END-ə-məl/ A type of neuroglial cell that forms the ependyma.

ependymitis /EP-end-ə-MY-tis/ Inflammation of the ependyma.

epiblast /EP-ə-blast/ The outer layer of cells on a blastoderm.

epicranium /EP-ə-CRAY-nee-əm/ The soft tissue overlying the cranium.

epidemic /EP-ə-DEM-mick/ (1) n. a situation in which an infectious disease, usually not of local origin, simultaneously affects an atypically large number of individuals; (2) adj. simultaneously infecting an unusually large number of individuals — epidemically /EP-ə-DEM-klee/

epidemiology /EP-ə-DEE-mee-ALL-ə-gee/ The study of the incidence of disease within populations and of optimal measures for its control.

epidermal /EP-ə-DERM-əl/ (also epidermic) Of, or pertaining to, epidermis.

epidermic /EP-ə-DERM-mick/ See: epidermal.

epidermis /EP-ə-DERM-əs/ (1) in plants: the dermal tissue system; (2) in animals: the outer layer of the skin.

epigenetics /EP-ə-jə-NET-ix/ The study of heritable changes in phenotype caused by mechanisms other than the alteration of nucleotide sequences.

epigenetic trait /EP-ə-jə-NET-ick/ A stably inherited phenotype resulting from changes in a chromosome without alterations in nucleotide sequences.

epiglottis /EP-ə-GLAWT-əs/ The flap that closes the glottis.

epididymis /EP-ə-DID-ə-məs/ A small structure attached to the testis, composed of a long, convoluted tube connecting the efferent ducts of the testis with the ductus deferens.

epididymoorchitis /EP-ə-DID-ə-mō-or-KĪ-təs/ Inflammation of the epididymis and testis

epilepsy /EP-ə-LEP-see/ A chronic neurological disorder characterized by periodic losses of consciousness, which may be accompanied by seizures.

epiphysis /eh-PIF-ə-səs/ A small bone separated from an larger bone by cartilage in early life, and that later fuses with the larger bone.

epiphyte /EP-ə-fight/ A plant that grows upon another plant (as on a tree) or on some object such as a rock.

epistasis The masking of one gene by another. For example, a gene causing baldness would mask the effect of a hair-color gene.

epithelial tissue (also ephithelium) A sheet of enclosing tissue. MORE INFORMATION

epithelialization The formation of epithelial tissue in a region where it was absent.

epoch A portion of a geological period. GEOLOGIC TIME SCALE

eponychia (sing eponychium) In the embryo, the horny structures from which the nails develop.

equid /EK-wid/ A member of the genus Equus.

equilibrium A state in which opposing forces are equal.

Equus /EK-wəs, EEK-/ The genus that includes horses, zebras, asses, and onagers. About zebra habitat

ER endoplasmic reticulum.

erratic /er-RAT-ick/ A large boulder left behind by a glacier, unlike other rock in its vicinity.

erbium (Er) metallic element; atomic weight 167.26; atomic number 68. PERIODIC TABLE

erectile Capable of becoming erect or stiff.

erg (1) the amount of work done by a force of one dyne acting through a displacement of one centimeter; (2) a desert region of shifting sands.

erythema /er-ri-THEE-mə/ Abnormal redness of the skin.

erythematic (also erythematous) /er-rith-ə-MAT-ik, er-ə-THEM-ə-təs/ Exhibiting or being similar to erythema.

erythemagenic /er-rith-ə-mə-JEN-ik/ Tending to produce erythema.

erythrocytes /er-RITH-rə-sights/ Red blood corpuscles, cells that function in transporting oxygen to the body's tissues.

erythromycin /er-RITH-rō-MĪ-sin/ An antibiotic derived from Streptomyces erythreus. Taken orally it kills many gram positive and some gram negative prokaryotes.

erythropia /ER-ə-THRŌP-ee-yə/ or erythropsia /ER-ə-THRŌP-see-yə/ A condition of the eyes in which objects are perceived as tinted red.

ES cells See: embryonic stem cells.

Escherichia coli /esh-ə-RICK-ee-yə KŌ-lī/ A rod-shaped bacterium found in the lower digestive tract. The most common bacterium used in genetic engineering due to its normal lack of pathogenicity, small genome size, and easy propagation.

esophageal /ə-SAWF-ə-GEE-əl/ Of or pertaining to the esophagus.

esophagectasis (also esophagectasia) /ə-SAWF-ə-jehk-TAY-səs, ə-SAWF-ə-jekt-TAY-zhə/ Dilation of the esophagus.

esophagostenosis /ə-SAWF-ə-gō-stə-NŌ-səs/ Narrowing of the esophagus.

esophagus /ə-SAWF-ə-gəs, ee-SAWF-/ The tube through which food and drink pass from the pharynx to the stomach; commonly known as the gullet. It has three coats: (1) an internal or mucous coat; (2) a middle or areolar coat; (3) an external or muscular coat. The muscular coat is made up of two thick layers of muscle fibers, an internal circular, and and external longitudinal layer. The areolar coat provides a loose connection between the internal and external coats. The mucous coat is thick and has longitudinal folds that disappear when the passage is distended with food.

essential amino acid An amino acid required in the diet of mammals.

EST Expressed sequence tag.

ester /ES-ter/ A compound formed by the combination of an alcohol with an organic acid.

esthesiology /es-theez-ee-AWL-ə-jee/ The scientific study of sensation — esthesiologist /es-theez-ee-AWL-ə-jəst, -jist/

esthesiometry /es-theez-ee-AWM-ə-tree/ The measurement of sensory discrimination.

esthesiophysiology /es-theez-ee-ō-fizz-ee-AWL-ə-jee/ Physiology of the senses.

estrogen /ES-trə-jen/ A hormone, produced by the ovaries, that prepares the uterus for embryo implantation.

estrogenic /es-trə-JEN-ick/ Causing estrus.

estrus (also estrum British: oestrus or oestrum) /ES-trəs, ES-trəm/ In female non-primate mammals, the periodically recurrent state of sexual receptivity ("heat").

ether /EE-ther/ An organic compound where an oxygen links two carbon chains.

ether, anesthetic /EE-ther/ Diethyl ether, the common general anesthetic and laboratory solvent.

ethmoid bone /ETH-moid/ A sievelike bone, filled with air cells, forming the roof of the nasal cavity and floor of the anterior portion of the braincase. LATERAL AND FRONTAL VIEWS | ROTATING VIEW

ethmomaxillary /ETH-mō-mack-SILL-er-ee/ Pertaining to the ethmoid bone and maxilla.

ethology /ee-THAWL-ə-jee/ The study of animal behavior — ethologist /ee-THAWL-ə-jist/

etiology (British: aetiology) /ET-ee-AWL-ə-jee, EET-/ The cause(s) of a disease; (2) the study of the causes of disease — etiological /ET-ee-ə-LAWJ-ik-əl, EET-/ etiologic /ET-ee-ə-LAWJ-ik, EET-/

eubacteria /yoo-back-TEER-ee-yə, -tin/ This term, formerly much in use, is now obsolete. MORE INFORMATION

euchromatin /YOO-krōm-ə-tən, -tin/ Chromatin that is uncoiled (not condensed) during interphase. Euchromatin reaches maximal condensation during mitotic metaphase.

eugenics /yoo-JEN-icks/ A political movement, without scientific foundation, promoting the idea that humanity can, and should be improved, by artificial selection; selective breeding of humans. The Nazis applied this pernicious policy with disastrous effect.

eukaryote /yoo-KARE-ee-yət, -ee-ote/ Organisms, both unicellular and multicellular, in which there is a membrane-bounded cell nucleus and other well-developed organelles. All organisms other than viruses and prokaryotes are eukaryotes. See also: prokaryote, chromosome. MORE INFORMATION

Euramerica (sometimes called "the Old Red Continent") /yer-am-MARE-ick-ə/ A supercontinent formed by the collision of Avalonia, Baltica, and Laurentia. It existed in the Late Silurian and in the Devonian. It later became part of Pangea, and still later, part of Laurasia. Today, the corresponding deposits, which are usually red in color, underlie Greenland and much of Europe and North America. Map

eurybathic /yer-ee-BATH-ik/ Bottom-dwelling, in either deep or shallow waters.

euryhaline /yer-ee-HAY-line, -HAL-/ Able to subsist in a broad range of salinities.

eurypterid /yə-RIP-ter-rəd/ A sea scorpion. Eurypterids, now extinct, were perhaps the most characteristic marine animals of the Silurian Period, a time when they were abundant and often gigantic (meters long). PICTURE 1 | PICTURE 2

eurythermal (also eurythermic) /yer-ee-THERM-əl/ Tolerating a broad range of temperatures.

eurytopic /yer-ee-TAWP-ik/ Tolerating a wide variation in one or more environmental factors.

eustachian tube /yoo-STAY-shən/ The narrow passage connecting the middle ear with the upper pharynx. PICTURE

eutelic /yoo-TELL-ick/ Having a fixed number of cells at maturity (as is the case in certain types of organisms such as rotifers and nematodes).

eutely /YOU-tə-lee, -də-/ The condition of being eutelic.

eutherian /yoo-THIR-ee-ən/ A member of the major division Eutheria of Class Mammalia, the other two major divisions being marsupials and monotremes. Eutherian mammals nourish their young within the uterus to mored advanced stage of development than do most marsupials.

eutrophication Superabundant growth of plants in lakes and rivers resulting from the release of nitrogen- and phosphorous-containing fertilizers. In the end this leads to accelerated decomposition of the dead plants by bacteria, which uses up oxygen, causing fish and other aquatic organisms to die of suffocation.

evaporite A deposit of minerals, such as anhydrite, gypsum, and halite, that remains after evaporation of seawater.

evolution /ev-ə-LOO-shən/ The production of new forms of life over time, as documented in the fossil record.

evolutionarily /ev-ə-loo-shə-NAIR-ə-lee/ With regard to, in connection with, or during the course of evolution.

evolutionarily conserved See: conserved sequence.

evolutionary /ev-ə-LOO-shə-nair-ee/ (also evolutional /ev-ə-LOO-shən-əl/) Connected with, pertaining to, dealing with, or involving evolution.

evolutionary biology /ev-ə-LOO-shə-nair-ee/ The branch of biology concerned with the study and identification of processes producing new forms of life.

evolutionism /ev-ə-LOO-shə-niz-əm/ The explanation of biological origins in terms of evolutionary processes.

evolutionist /ev-ə-LOO-shə-nəst/ An adherent or proponent of evolutionary theory.

excision /ex-SIZ-shən, ex-SISH-/ Cutting out or away, removal by cutting.

excitomuscular /ex-SEET-ō-MƏSK-yə-lər/ adj. Stimulating or causing muscular activity.

excrement /EX-krə-ment/ n. Feces.

excreta /ex-KREE-tə/ n. (1) in a medical context: feces; (2) more broadly, any waste product of the body or of a cell.

excretion /ex-KREE-shən/ n. The process of expelling waste materials from the body, or from an individual cell.

exergonic /ex-er-GAWN-ick/ adj. Pertaining to a spontaneous chemical reaction that does not absorb free energy. Compare: endergonic

exine /EX-seen, -īn/ n. The outer of the two coverings of a pollen grain or spore. It has a characteristic surface patterning and shape specific to each type of pollen or spore, which is used in palynology.

exocrine gland /EX-ə-krin/ n. A gland that secretes its contents onto an epithelial surface (and not into the bloodstream or lymphatic system).

exocytosis /ex-ō-sī-TŌ-səs/ The expulsion of macromolecules from a cell. Compare: endocytosis MORE ABOUT EXOCYTOSIS

exogenesis /EKS-ō-GEHN-ə-səs/ One of the two major hypotheses concerning the origin of life on Earth. It proposes that life on this planet arrived from elsewhere in the universe, presumably in the form of frozen particles containing prokaryotes and, perhaps, other simple organisms. This hypothesis is sometimes called panspermia, but this latter term, strictly speaking, refers to the claim that the "seeds of life" are spread throughout the universe and begin to develop wherever suitable environments exist. The other major hypothesis, opposed to exogenesis, is primordial-abiogenesis.

exogenous virus /ehks-AW-jehn-əs/ A virus of contagious origin, not inherited from a parent.

exon n. A protein-coding portion of a gene. See also: intron.

exonuclease /EKS-ō-N(Y)OOK-lee-aze/ n. An enzyme that digests DNA, beginning at the end (as opposed to an endonuclease).

exosepsis n. Sepsis of external origin.

exoskeleton n. A hard case on the external surface of many invertebrates, such as the shell of a mollusk or the cuticle of an insect.

exotic /eks-AWT-ick/ (1) adj. nonnative to a given geographic region; (2) n. an organism or disease not normally occurring in a particular geographic region. Compare: endemic

expectorant /ek-SPECK-tə-runt/ n. A drug that thins mucus in the respiratory tract and that therefore eases breathing difficulties. Guaifenesin is the most common expectorant. Compare: antitussive

exponential growth /exs-pō-NENCH-əl/ n. Growth in which a population increases by a fixed multiple after the passage of a time interval of a specified length (e.g., doubles every ten years).

exponential growth phase /exs-pō-NENCH-əl/ See: logarithmic growth phase.

expressed gene See: gene expression.

expressed sequence tag (EST) A short portion of a cDNA molecule serving to identify a gene. Used in locating genes on chromosomes. See also: sequence tagged site.

exteroceptor /ex-TARE-ō-sep-tər/ n. A sensory organ, as the eye, that receives external stimuli.

extradural /EX-trə-DƏR-əl/ adj. Outside the dura mater.

extremophile /ig-STREM-ə-file/ n. An organism that thrives under extreme conditions, such as extreme heat or salinity.

exude /ig-Z(Y)OOD/ v. To pass or ooze through a tissue (said of fluids).

exuvia /ig-ZOO-vee-ə, eg-/ (pl exuviae /ig-ZOO-vee-ee, eg-/) n. The sloughed outer covering of an animal, especially an invertebrate. — exuvial /ig-ZOO-vee-əl/ PICTURE OF SCORPION EXUVIA | PICTURE OF DRAGONFLY EXUVIA | PICTURE OF TARANTULA EXUVIA

exuviate /ig-ZOO-vee-ate/ v. To molt, to shed an exuvia. — exuviation /ig-ZOO-vee-ay-shən/

Human eye (Enlarge Image)

eyespots /Ī-spawts/ n. Light-sensing organelles present in certain microorganisms.

eye n. Visual organ; in humans it has three layers or "coats" — (1) the retina; (2) the uvea; (3) the sclera and cornea. MORE INFORMATION

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