520 GLOSSARY 



Nephridium (ne frid'i um), a form of excretory organ, as found in the earth- 

 worm. 



Nephrostome (nef'ro stom), the funnel-shaped aperture at the medial end of a 

 nephridium. 



Neural (nu'ral), pertaining to the nervous system or to a nerve. 



Neurilemma (nu rilem'a), the membranous outer coat of a nerve fiber. 



Neurold transmission (nu'roid), primitive transmission of impulses from cell to 

 cell. 



Neuron (nu'ron), a nerve cell together with its processes. 



Notocliord (no'tokord), a flexible rod extending anterior to posterior in the 

 longitudinal axis of the body dorsal to the digestive tube and ventral 

 to the nerve cord in chordates. 



Nocturnal (nok tur'nal), reference to night. Contrasted to diurnal which per- 

 tains to daytime. 



Nodes of Ranvier (ranvya'), constrictions in meduUated nerve where the myelin 

 sheath is interrupted. 



Nomenclature (no'men kla ttir), a system of naming objects or ideas. 



Nondisjunction (non dis jungk'shun), the failure of homologous chromosomes to 

 separate after synapsis and both go to one daughter cell with none to the 

 other. 



Nucleolus (nu kle'6 Ids) (Plasmosome), a body within the nucleus containing 

 material that is not chromatin. 



Nucleus (nti'kleus), a typically spherical body within the cell that contains the 

 chromatin. 



Nymph (nimf), the larval stage of an insect which undergoes incomplete meta- 

 morphosis; also the larval stage of a few vertebrates. 



Ocellus (6 sel'us), a simple type of eye, as in some insects. 



Ommatidium (6m a tid'i um), one of the numerous rodlike units of the com- 

 pound eye. 



Ontogeny (ontoj'eni), the entire development and life history of an individual 

 organism. 



Oocyte (o'o sit), the female germ cell before maturation is completed. 



Oogenesis (o 6 jen'e sis), the maturation of the female germ cell. 



Oogonium (6 6 go'nium), the female germ cell during the multiplication and 

 growth stages of maturation. 



Operculum (6 pur'kuliim), a fold of skin, bone, and scales, which covers the 

 gills of fishes and certain Amphibia; also the bony structure closing the 

 aperture of certain snail shells. 



Organ (or'gan), an arrangement of two or more tissues as a part of the body 

 which performs some specific function or functions. 



Organism (or'gan iz'm), any independent living being. 



Orthogenesis (or tho jen'e sis), the theory which holds that animals tend to 

 develop along lines leading constantly in the same direction because 

 they are determined by internal factors. 



Osmosis (osmo'sis), diffusion of substances dissolved in fluid, through a semi- 

 permeable membrane. 



Ossicle (os'ik'l), a small bony structure. 



Ostium (os'tium), a mouthlike opening or entrance. 



Otocyst (o'tosist), the primitive organ of hearing. 



Ova (6'va), mature female germ cells. Sing., ovum (o'vum). 



Ovary (6'vari), the female gonad. 



Oviduct (6'vidukt), the duct for the passage of ova from the ovary to the 

 exterior of the animal. 



Oviparous (6 vip'a riis), pertaining to those animals which lay eggs that hatch 

 after exclusion from the body. 



Ovipositor (o vi poz'i ter), an organ of female insects and others which serves 

 in helping to deposit the egg. 



