472 GENERAL ZOOLOGY 



Nerve. A bundle of nerve-cell processes, axons, or dendrons, or both. 



Neural canal. The cavity in the vertebral column containing the spinal 

 cord. 



Neural tube. The embryonic ectodermal tube from which the brain and 

 spinal cord develop. 



Neuron. A nerve cell, including cell body, axon, and dendron. 



Nitrogen equilibrium. A condition in which the animal body is receiving 

 from food as much protein nitrogen as it is metabolizing and eliminating. 



Notochord. A structure characterizing the phylum Chordata consisting 

 of a cylindrical rod lying ventral to the neural tube. In all vertebrates 

 except Cyclostomata it is replaced in varying degrees by the centra of 

 the vertebral column. 



Nucleolus. A usually spherical body within the nucleus taking an acid 

 stain. Chromatin takes the basic stain. 



Nucleus. One of the two principal components of a cell, usually occu- 

 pying a central position in the cytoplasm. 



Olfactory. Pertaining to the sense of smell. 



Ontogeny. The development of the individual. 



Oocyte. The developmental stage of the egg during the growth period. 



Oogenesis. The development of a mature egg from a primordial germ cell. 



Oogonium. One of the products of division of the female primordial germ 

 cell. 



Oosperm. A fertilized egg. 



Ootid. The final product of oogenesis, the mature egg. 



Order. A subdivision of a class in classification. 



Organ. A tissue complex which performs a definite function. 



Organelle. A protozoan organ; an organ within a cell. 



Organic. Of or pertaining to organisms. 



Organism. A living thing. 



Organology. The study of organs. 



Osmosis. The slow passage or diffusion of fluids through semipermeable 

 membranes. Osmotic pressure results from the difference in behavior 

 of solvent and solute with respect to the membrane, the latter not being 

 equally permeable to both. The surfaces of cells constitute the mem- 

 branes through which substances must pass in and out of cells, the 

 direction being determined by conditions within the cell with reference 

 to the surrounding medium. 



Osseomucoid. A glycoprotein found in bone. 



Ovary. The organ in which eggs develop. 



Oviparous. Egg laying. 



Ovum. Egg, female gamete. 



Oxidation. The chemical combining of a substance with oxygen, partial 

 or complete. Combustion. 



Paedogenesis. Reproduction in the larval state. 



Paleontology. The science of prehistoric organisms. 

 Parasitism. An animal association in which one member, the parasite, 

 derives nourishment from the tissues of the other, the host, to the 

 detriment of the latter. 



