GLOSSARY 453 



Neural crest. Two ribbons of ectodermal cells on either side the primi- 

 tive neural tube of the vertebrate embryo; some of these cells be- 

 come the ganglion cells of the dorsal roots of the spinal nerves; 

 others form the cells of the sympathetic system; still others are 

 concerned in the formation of certain endocrine glands. 



Neuroid transmission. The transmission of impulses by tissues other 

 than nerves. 



Neurology. The science that deals with the nervous system of animals. 



Neuromotorium. a small body in the protozoon, Diplodinium, which 

 appears to be a regulating centre in its coordination mechanism. 



Neuron. The nerve cell. 



Nitrogen-fixing bacteria. A type of bacteria that in their metabolic 

 processes combine the nitrogen from the air with elements in the 

 soil, for example, potassium, to form inorganic nitrogen compounds. 

 Nitrogen-fixing bacteria commonly inhabit nodules on the roots of 

 plants, especially clovers, beans, etc. 



Notochord. An axis of the chordate body consisting of a rod of cells in 

 the body wall dorsal to the ccelom. 



Nuclear membrane. A thin membrane surrounding the cell nucleus. 



Nucleo-proteins. Proteins characteristic of the cell nucleus. They are 

 complex molecules containing nucleic acid and appear to be essen- 

 tial to life, since all cells contain them. 



Nucleus. A more or less spherical body of specialized protoplasm that 

 occurs in all cells and without which a cell cannot exist. 



Nutrition. The process of obtaining and utilizing foods. 



Obligatory anaerobe. An organism that cannot live in the presence of 

 atmospheric oxygen. 



Ontogeny. The process of development of a new individual. 



Oogenesis. The process of conversion of the primordial female germ cell 

 into a mature ovum. 



OoGONiA. An early stage in the maturation of the tgg, during which 

 the germ cells increase in number by ordinary mitotic divisions. 



Organ. A special part that performs a particular function. For example, 

 the human stomach is an organ concerned in digestion; the flagellum 

 of a mastigophoron is its organ of locomotion. 



Organ of Corti. The structure in the inner ear which bears the nerve 

 endings that receive sound vibrations. 



Organ system. An assembly of organs which performs a general func- 

 tion; for example, the teeth, salivary glands, oesophagus, stomach, 



