GLOSSARY 445 



NITROGEN-FIXING BACTERIA. Types of Bacteria which take free 

 atmospheric nitrogen and combine it with oxygen so that nitrates 

 available for green plants are formed. Found in the soil and 

 in tubercles on rootlets of various leguminous plants. 



NOTOCHORD. An axial cord of cells about which the backbone is 

 formed. Gradually replaced by the centra of the vertebrae in 

 the ascending series of Vertebrates. 



NUCELLUS. The megasporangium of Flowering Plants. See Ovule 

 and Embryo sac. 



NUCLEOLUS. A spherical body of achromatic material within the 

 nucleus. Plasmosome. See Karyosome. 



NUCLEUS. A specialized protoplasmic body in all typical cells. 

 Most characteristic element is chromatin. 



OESOPHAGUS. Narrow tube leading from pharynx to stomach. 



OLFACTORY. Relating to the sense of smell. 



ONTOGENY. The developmental history of the individual. See 

 Phylogeny. 



OOCYTE. The ovarian egg before maturation. 



OOGENESIS. The development of the mature egg from a primordial 

 germ cell. 



OPTIC LOBES. Thickenings of the dorsal surface of the mid-brain. 



ORDER. In classification, a main subdivision of a class. See Family. 



ORGAN. A complex of tissues for the performance of a certain func- 

 tion; e.g., the heart. 



OSMOSIS. Diffusion of dissolved substances through a semi-perme- 

 able membrane. Osmotic pressure may be considered as a result 

 of the inhibited power of diffusion of a dissolved substance in- 

 hibited because the membrane is semi-permeable. The physical 

 phenomena of diffusion and osmosis are complicated in living 

 cells by the fact that their limiting surfaces may function now as 

 permeable and again as semi-permeable membranes, i.e., per- 

 mitting water but not the substance in solution to pass 

 through. 



OSTEOLOGY. The study of the Vertebrate skeleton. 



OVARY. The definitive female reproductive organ in which the 

 gametes (eggs) develop. 



