GLOSSARY 391 



Nervous tissue. Tissue capable of transmitting impulses; as the tissues of the brain, 



spinal cord, and nervos. 

 Net knot. A thickened portion of the chromatin of a cell nucleus. 

 Neural arch. That part of a vertebra above the centrum and neural canal. 

 Neural canal. The opening in a vertebra through which the spinal cord extends. 

 Neural crest. One of a number of groups of cells at the sides of the brain and spinal 



cord of an embryo, from which ganglia and nerves are developed. 

 Neural fold. One of the ridges of ectoderm forming the earliest development of the 



nervous system. 

 Neural groove. An elongated depression between the neural folds of an embryo. 

 Neural spine. A projection rising from the middle of the neural arch of a vertebra. 

 Neural tube. The tube formed beneath the ectoderm by the union of the neural 



folds along their crests. 

 Neurilemma. The thin cellular covering of a nerve fiber. 

 Neuromuscular. Combining the functions of contraction and the transmission of 



impulses. 

 Neuron {nu' rone). A nerve cell. 

 Neutron. A particle, like a proton but without electric charge, entering into the 



composition of the nuclei of most atoms. 

 Niacin (nz' a sin). The antipellagra vitamin, part of the B complex. 

 Nicotinic acid {nik' o tin' ik). Same as niacin. 



Nomenclature {no' men kla' lure). A system of naming; terminology. 

 Nostril. One of the external openings of the nasal chamber. 

 Notochord (no' to kord). A cylindrical rod of cells beneath the nervous system of an 



embryo (adult of some animals). It is the forerunner of the spinal column of the 



vertebrate animals. 

 Notophthalmus {no' tof thai' mus). A genus of salamanders. 

 Nuchal plate {nu' kal). In turtles, the median plate of the carapace at the anterior 



end. 

 Nuclear membrane. A thin film of protoplasm surrounding the nucleus of a cell. 

 Nuclear sap. The liquid forming the bulk of the nucleus of a cell. 

 Nucleolus {nu kle' o lus). A small, usually rounded body found in the nuclei of many 



cells, which is of different chemical composition from the rest of the nucleus. 



Its function is uncertain. 

 Nucleus. A highly refractive, deeply staining body of specialized protoplasm found 



within nearly all cells. 

 Nudibranch {nu' di brank). One of a group of marine moUusks. 



Obelia. A genus of hydroids, or colonial hydralike animals of the phylum Coelen- 



terata. 

 Octopus. A genus of devilfishes (moUusks) having eight arms. 



Oenothera (e' no the' ra). A genus of plants to which the evening primroses belong. 

 Oken, Lorenz. German naturalist and transcendentalist philosopher, 1779-1851. 

 Olfactory. Pertaining to the sense of smell. 



Oligocene. Of early Tertiary time, between Eocene and Miocene. 

 Oligochaeta {ol' i go ke' to). A subclass of Chaetopoda (Annelida), including chiefly 



terrestrial and fresh-water worms with relatively few setae which do not rest on 



fleshy outgrowths but project directly from the body wall. The earthworm is an 



example. 

 Onychophora {on' i kof o ra). A class of primitive Arthropoda having tracheae and 



one pair of antennae. Peripatus is an example. 



