duodenum (doo-o-dee'num) : first part of the 

 small intestine. 



eardrum: membrane lying between the middle ear 

 and the canal of the outer ear; it transmits 

 vibrations to the small bones of the middle 

 ear. 



echinoderms (eh-kine'o-derms) : members of a 

 phylum of marine invertebrates with radial 

 symmetry and spiny coverings; starfish group. 



ecology (ee-kol'o-gee) : that field of biology that 

 deals with the relationship of organisms to 

 their physical surroundings and to other or- 

 ganisms. 



ectoderm (ek'toe-derm) : outermost layer of cells 

 in the hollow cup stage of an animal embryo. 



effector (ef-fek'tor) : an organ, tissue, or cell in 

 an animal that responds to a stimulus, such 

 as a muscle or gland. 



egg: structure with more or less substantial cover- 

 ing laid by the female of many kinds of ani- 

 mals, containing usually a fertilized egg cell. 

 Egg cell: female sex cell or ovum before fer- 

 tilization in plants and animals. 



electron microscope: microscope of extremely 

 high power, using beams of electrons instead 

 of rays of light and recording the image on a 

 photographic plate or fluorescent screen. 



element: substance of which the molecules con- 

 tain one kind of atom; at present 98 elements 

 are recognized. 



embryo (em'bree-oh) : animal in the early stages 

 of its development before it is hatched or 

 born; in a plant, before germination. 



embryo sac: large cell in the plant ovule within 

 which nuclear divisions result in the forma- 

 tion of the egg cell nucleus and other nuclei. 



emotions (ee-moh'shuns) : feelings of joy, sorrow, 

 hate, and the like which involve important 

 changes in the action of some internal organs 

 and even some skeletal muscles. 



emulsion (ee-mul'shun) : condition of an oil in 

 which it exists in tiny droplets, each sur- 

 rounded by a thin film which gives the oil a 

 milky appearance. 



enamel: lifeless, hard, glossy outer layer of the 

 crown of a tooth in most mammals and many 

 other vertebrates. 



end brush: branched endings of the axon through 

 which a stimulus passes into the dendrites of 

 another neuron or into an effector. 



endoderm (en'doh-durm) : innermost layer of 

 cells in the hollow cup stage of an animal 

 embryo. 



endosperm: group of food-storing cells outside the 

 embryo in the seeds of some kinds of plants 

 (mostly monocots). 



energy: the ability to do work. 



entomology (en-toe-mol'o-jee) : study of insects. 



environment (en-vie'run-ment) : all the surround- 

 ings of a living organism, including other or- 

 ganisms. 



enzyme: substance made by living cells which 



brings about or hastens a chemical change, 

 without being itself permanently affected; 

 many enzymes are digestive, such as pepsin 

 and ptyalin; others hasten oxidation, etc. 



eohippus (ee-oh-hip'pus) : small extinct animal 

 believed to be the ancestor of the modern 

 horse. 



epidermis (ep-i-der'mis) : outermost layer or lay- 

 ers of cells in an animal or plant. 



epiglottis (ep-ee-glot'tis) : flap of tissue that is 

 folded down over the top of the windpipe 

 (voice box) during swallowing. 



epithelial (ep-e-thee'lee-al) tissue: covering tissue 

 in plants and animals; includes lining tissues 

 in animals. 



era (ear'a) : a major division of time in the his- 

 tory of the earth; many geologists divide time 

 since the beginning of the earth into seven 

 eras. 



ergosterol (er-goss'ter-ohl) : substance produced 

 by and found in many plants, which can be 

 turned into vitamin D by ultraviolet rays. 



erosion (e-row'shun) : wearing away the surface 

 of the earth by agents such as water and gla- 

 ciers. 



essential organs: organs within a flower directly 

 concerned with sexual reproduction; stamens 

 and pistils. 



eugenics (you-jen'ics) : improvement of the hu- 

 man race by breeding. 



Eustachian (you-stake'ee-an) tube: tube connect- 

 ing the middle ear with the throat. 



euthenics: improvement of the human race 

 through improvement of the environment. 



evolution: change or development. Organic evolu- 

 tion: theory that all living things are de- 

 scended from earlier forms, that the first or- 

 ganisms to appear on the earth were simple, 

 and that they gave rise throughout the ages 

 to more and more complex forms. 



excretion: giving off wastes formed in oxidation 

 by animal or plant bodies or cells. 



exoskeleton (ex-oh-skel'e-tun) : a hard protective 

 covering, as in arthropods. 



family: in classification the major subdivision of 

 an order, commonly composed of a number of 

 genera. 



fang: sharp, hollow, or grooved tooth by which 

 venom is injected by poisonous snakes. 



feeble-minded: having so low an intelligence that 

 the person is unable to meet the conditions of 

 life satisfactorily. 



fermentation: changing of sugar by some micro- 

 organisms, especially yeasts, into alcohol and 

 carbon dioxide; energy is released. Also 

 changing of other organic compounds by mi- 

 croorganisms, with production of a gas. 



ferns: plants of many species belonging to the 

 pteridophyte group, with true leaves, roots, 

 and horizontal stems with conducting tissue 

 much like thai in higher plants; spores borne 

 on leaves or modified leaves. 



588 



