846 Appendix 



Nucellus (nu-sel'us) (L. nux, nut), the megasporangium of an ovule, locatec 



inside the integument and enclosing the megagametophyte. 

 Nuclear electron, one within the nucleus of the atom. 

 Uucleolus (nu-kleolus) (L. dim. of nucleus), the somewhat spherical body 



within the nucleus, probably of regulatory function (same as Plasmo- 



some). 

 Nucleoplasm (nu' kle o plazm) (L. nux, nucleus; Gr. plasma, liquid), Hquid part 



of the nucleus. 

 Nucleus (nu'kleus) (L. nucleus, kernel), specialized, central, organized structure 



in most cells. 

 Nymph (nimf) (Gr. nymphe, bride), specific stage in metamorphosis of such 



insects as the grasshopper. 



O 



Obligate (ob'ligate) (L. ob, about; ligo, bind), unable to change life habits to 



suit varying conditions. 

 Occipital (ok -sip' i tal) (L. occiput, back of head), base of skull. 

 Ocellus (plural ocelli) (o -sel' us) (L. o cuius, eye), simple eye. 

 Octopus (ok' to pus) (Gr. okta, eight; pous, feet), a mollusk with eight feet 



(arms) . 

 Oculomotor (ok u lo -mo' ter) (L. ocidus, eye; movere, to move), moving the eye. 

 Oesophagus (esophagus) (e-sof'agus) (Gr. oise, bear; phagein, to eat), tube 



from pharynx to stomach. 

 Olfactory (ol -fak' to ri) (L. olere odor: facere, to make) pertaining to odors. 

 Ommatidium (om a -tid' i um) (Gr. omma, eye), unit of which compound eyes 



are made, as in crayfish, certain insects. 

 "Oninis cellula a cellula," Virchow's dictum that all cells arise from cells. 

 Omnivorous (om -niv' o rus) (L. omnis, all; vovare, to eat), eating both plant 



and animal tissues. 

 Ontogeny (on -toj' e ni) (Gr. on, being; genes, born), developmental life history 



of an individual, including embryology, metamorphosis, and adolescence, 



as distinguished from phylogeny (evolution of a race or group). 

 Oocyte (o'osit) (Gr. oon, egg; kytos, cell), female egg before maturation. 

 Oogamy (o -og' a mi) (Gr. oon, egg; gamos, marriage), union of nonmotile egg 



and male gamete. 

 Oogenesis (o o -jen' e sis) (Gr.oon, egg; genesis, origin), formation of an egg 



and its preparation for fertilization and development. 

 Oogonium (oo-go'nium) (Gr. oon, egg; gonos, offspring), primordial egg cell 



before maturation; the one-celled female sex structure in certain thallo- 



phytes and produces one or more eggs. 

 Operculum (o -pur' ku lum) (L. operculum, cover or lid), lidlike covering. 

 Opsonin (op' so nin) (Gr. opsonein, to cater), substance in the blood which aids 



phagocytes to destroy bacteria. 

 Optic (op' tik) (Gr. optikos, sight), pertaining to sight or the eye. 

 Order (or' der) (L. ordo, order), methodical arrangement; group of closely allied 



organisms all belonging to the same class. 

 Organ (or' gan) (Gr. organon, an implement), a group of different tissues all 



performing a common function, 



