GLOSSARY 505 



Nitrifying Bacteria. Nitrite Bacteria which, in the process of their 

 nutrition, change ammonia (NH 3 ) into compounds with the N0 2 radical 

 (nitrites), and Nitrate Bacteria which change nitrites into compounds 

 with the N0 3 radical (nitrates). 



Nitrogen-fixing Bacteria. Types of Bacteria which take free atmos- 

 pheric nitrogen and combine it with oxygen so that nitrates available 

 for green plants are formed. Found in the soil and in tubercles on root- 

 lets of various leguminous plants such as Beans, Clover, Alfalfa. 



Non-disjunction. Failure of homologous chromosomes to separate after 

 synapsis. Therefore they are not independently segregated during 

 maturation — both pass to the same gamete. 



Notochord. An axial cord of cells characteristic of the Chordates, and 

 about which the vertebral column is formed in Vertebrates. 



Nucleolus. A spherical, achromatic body in the nucleus. Plasmosome. 

 Karyosome is chromatic. 



Nucleus. A specialized protoplasmic body in all typical cells. Most 

 characteristic element is chromatin. See Cytoplasm. 



Ocellus. Sense organ responsive to light, especially the simple eyes of 

 Insects. -See Compound Eye. 



Olfactory. Relating to the sense of smell. 



Ontogeny. The developmental history of the individual. See Phylogeny. 



Oocyst. Encysted zygote; e.g., of Malarial Parasite. 



Oocyte. The ovarian egg before maturation. 



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

 cell. 



Optic Lores. 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 function; 

 e.g., the heart. 



Osmosis. Diffusion of dissolved substances through a semi-permeable 

 membrane. Osmotic pressure may be considered as a result of the in- 

 hibited power of diffusion of a dissolved substance — inhibited because 

 the membrane is semi -permeable, i.e., permitting water but not the 

 substance in solution to pass through. The physical phenomena of dif- 

 fusion and osmosis are complicated in living cells by the fact that their 

 limiting membranes undergo changes in permeability. See Diffusion. 



Osteology. The study of the Vertebrate skeleton. 



Ostium. In Sponges, the opening from the gastral cavity to the exterior. 



Oviparous. Egg-laying. .See Viviparous. 



Ovum. Egg. Female gamete. 



Oxidation. The combination of any substance or its constituent parts 

 with oxygen. Combustion. 



