828 Appendix 



Division of labor, distribution of functions among cells, organs, or individuals. 

 Dizygotic (di zi -got' ik) (Gr. di, two; zygon, yoke, pair), derived from two eggs 



as in certain types of twins (contrast with Monozygotic). 

 Dominance (dominant character), one of a pair of alternative characters which 



is always expressed when its gene is present and which appears to exclude 



the other (recessive) character. 

 Dorsal (dor' sal) (L. dorsum, back), the back side of higher animals. 

 Dorsal aorta, chief artery arising from the heart to distribute blood to the body. 

 Dorsal horn (root), sensory root of a spinal nerve carrying impulses into the 



spinal cord; ventral root carries impulses from the cord. 

 Dorso ventral differentiation, body with definite dorsal (back) and ventral (belly) 



sides or regions. 

 Drosophila (dro -sof i la), the common fruit or banana fly; used extensively for 



heredity experiments. 

 Ductless glands, see Endocrine glands. 

 Duodenum (duo-de'num) (L. duodeni, twelve), anterior part of small intestine, 



twelv-e fingerwidths long. 

 Duplicate factors, different factors (genes) having identical but not cumulative 



effects. 



E 

 Ecdysis (ek'disis) (Gr. ek, out; dyein, to come), the losing or molting of an 



outer structure as in the crayfish, insects, etc. 

 Echinodermata (e kin o -dur' ma ta) (Gr. echinos, spiny; dermos, covering), spiny- 

 covered animals, such as starfishes, sea urchins, sand dollars, etc. 

 Ecology (e-kol'oji) (Gr. oikos, home; logos, study), scientific study of living or- 

 ganisms and their living and nonliving environments. 

 Ectoderm (ek' to durm) (Gr. ektos, outside; derma, skin), outer layer of germ 



cells. 

 Ectoparasitism (ek to -par' a sit -izm) (Gr. ektos, outside; para, beside; sitos, food), 



parasites attached externally to the host. 

 Ectoplasm (ectosarc) (ek' to plazm) (Gr. ektos, outside; plasma, liquid), outer 



layer of cell cytoplasm. 

 Efferent (ef er ent) (L. ex, out; ferro, to carry), convey away from. 

 Egest (e-jest') (L. ex, out; gerere, to carry) to throw out, usually indigestible 



material. 

 Egg (ovum), the mature female sex cell of a plant or animal. 

 Elater (el'ater) (Gr. elater, driver), a springlike organ of various plants to 



disperse spores. 

 Electrolyte (e -lek' tro lit) (Electric; Gr. lytos, dissolved), a substance, such as 



salts, acids, and bases, which in solution dissociates into electrically charged 



ions. 

 Electrolytic dissociation, breaking up of the molecules of electrolytes (acids, bases, 



salts) into electrically charged positive and negative ions capable of con- 

 ducting an electric current. 

 Electron (e -lek' tron) (Gr. elektron, gleaming, sun), smallest part, or unit, of 



negative electricity {see Nuclear electrons and Extranuclear electrons). 

 Element (el' e ment) (L. elementum, unit), a substance whose atoms are all the 



same; over ninety elements are known. 



