602 GLOSSARY 



nuclear sap. The more fluid part of a nucleus; also called karyolymph. 



nucleolus (nu kle' 6 lus). In the nucleus a sharply defined body the nature and 

 function of which vary in different nuclei and in many cases are unknown. 



nucleoplasm (nu' kle 6 plaz'm). The protoplasm contained in the nucleus, as 

 distinguished from cytoplasm. 



nucleus (nu' kle us). A portion of the protoplasm of a cell which is set off by a 

 membrane and which contains the chromatin and also produces enzymes that 

 stimulate the activities of the cytoplasm; adj., nuclear. 



nurse cell. An egg cell which contributes its substance to another egg cell, which is 

 developing, and therefore does not itself produce an embryo. 



nymph (nimf). The immature stage of an insect which undergoes incomplete meta- 

 morphosis; also the resting stage in the development of certain other inverte- 

 brates, coming in between the larva and the adult, as in certain mites; adj., 

 nymphal. 



Ocellus (o sgr us). A simple eye, as in an insect. 



-old. G. ; like, resembling. 



olfactory (ol fak' to rl). Pertaining to the sense of smell. 



ommatidium (6m a tid' i lim). A rodUke unit in a compound eye. 



ont (ont). G.; a being. 



ontogeny (6n t6j' e nl). The development of the individual from the egg cell to the 



adult condition; adj., ontogenetic (6n to je n6t' Ik). 

 00 (o' 6). G.; egg. 



oocyte (6' 6 sit). An egg cell during the maturation period in oogenesis. 

 oogenesis (6 6 j6n' e sis). The development of a female germ cell from the primordial 



sex cell to the mature egg cell. 

 oogonium (o 6 go' nl um). The female sex cell during the multiplication and growth 



periods in oogenesis. 

 operculum (6 pur' ku lum). A fold of skin covering the gills of an amphibian larva; 



a similar fold, containing scales, covering the gills of fishes; a horny or Hmy plate 



closing the opening of a snail shell; and other structures which are hdlike. 

 optic (op' tik). Pertaining to the eye. 



optimum (op' tl mum). The most favorable condition; adj., optimal, 

 oral. Pertaining to the mouth, 

 organ. An assemblage of tissues all contributing to the performance of some 



function. 

 organic (or gUn' Ik). That which relates to living things, or has been so related. 

 organism (or' gin Iz'm). A mass of living matter capable of maintaining independent 



existence and all parts of which contribute more or less to the activities of the 



whole. 

 organogeny (or ga noj' e nl). The development of organs in embryogeny. 

 ortho- (or' tho). G.; straight. 

 orthogenesis (or tho jen' e sis). The theory that animals tend to develop along lines 



leading constantly in the same direction and which are determined by internal 



factors. 



Osbom, Henry F. American paleontologist, at American Museum of Natural 

 History, New York; 1857-1935. 



osmosis (6s mo' sis). The passage of miscible fluids through a semipermeable mem- 

 brane, usually from a region of higher concentration to one of lower concentration. 



osmotic (6s m6t' Ik) pressure. The unbalanced pressure due to differences of con- 

 centration in solutions which are on opposite sides of a semipermeable membrane. 



ossicle (6s' I k'l). A small bone. 



oste (6s' t6). G.;bone. 



oto (o' t6). G.; referring to the ear; adj., otic (6' tIk). 



