GLOSSARY OF IMPORTANT TERMS 



681 



between the cerebellum and the 

 spinal cord. 



Medullary rays : thin plates of pith 

 which separate the wood of dicoty- 

 ledonous stems into wedge-shaped 

 masses. 



Mesoderm (mes'6 durm) : the middle 

 layer of cells in a young animal 

 embryo. 



Metabolism (me tab'6 liz'm) : changes 

 taking place continually in living 

 cells which may result in either 

 building up or breaking down the 

 cells. 



Metamorphosis (met'd mor'fo sis) : 

 change of form undergone from egg 

 to adult, as in insects. 



Micronucleus (mi'kro nu'kle us) : the 

 small nucleus in a Paramecium. 



Micropyle (mi'kro pil) : the hole 

 where the pollen tube enters the 

 embryo sac. 



Midrib : central vein of a leaf. 



Migrant (mi'grant) : an animal which 

 moves from one place to another 

 and back regularly at stated sea- 

 sons of the year. Many birds mi- 

 grate to warmer regions for the 

 winter. 



Mimicry (mim'Ik ri) : the imitation in 

 form or color of a harmful insect by 

 a harmless one which is protected 

 thereby. 



Mitosis : a complex type of cell divi- 

 sion, characterized by an equal dis- 

 tribution of chromatic material. 



Molecule (mol'e kul) : unit of a 

 chemical substance. 



Mollusca : the phylum of animals to 

 which the clam belongs. 



Monocotyledon (mon'6 kot'I le'd&n) : 

 a plant that bears seeds having but 

 one cotyledon. 



Monoecious (mo ne'shfts) : having 

 staminate and pistillate flowers on 

 the same plants. 



Motor neuron : a neuron, or nerve 

 cell, the branches of which end in 

 an effector (muscle or gland) which 

 brings about activity. 



Mucous (mu'k&s) membrane : a deli- 

 cate, moist membrance lining all 

 body passages which have an exter- 

 nal opening. 



Muscle (mus"l) : a contractile tissue , 



capable of bringing about move- 

 ment. 



Mutation (mtl ta'sbun) : a heritable 

 modification arising from internal 

 causes in an organism. 



Mycelium (ml se'll iim) : the thread- 

 like body of a mold, or other fungus. 

 The individual threads are called 

 hyphae. 



Myriapoda (mir'I ap'6 da) : class of 

 animals to which centipedes and 

 millipedes belong. 



Narcotic (nar kot'ik) : a substance 

 which blunts the senses and in large 

 quantities causes insensibility. 



Nectar (nek'tdr) : a sweet fluid se- 

 creted by certain groups of cells 

 known as nectar glands in a flower. 

 From this substance bees make 

 honey. 



Nemathelminthes (nem'a thel mm'- 

 thez) : a phylum of animals to 

 which the unsegmented round- 

 worms belong. 



Neuron (nu'ron) : a nerve cell and its 

 branches. 



Nitrate (ni'trat) : a soluble salt of 

 nitric acid. 



Nitrogen (ni'tro jen) : a gaseous ele- 

 ment, found in many organic com- 

 pounds and forming almost four 

 fifths of the atmosphere. 



Nitrogen-fixing bacteria : bacteria 

 which take free nitrogen from the air 

 in the soil and build into nitrites 

 which are later converted into ni- 

 trates. These nitrates can be used 

 by the plants. 



Nucleus (nu'kle us) : the center of 

 activity in the living cell. 



Nutrient (nti'trl ent) : nourishing sub- 

 stance contained in foods. 



Nutritive (nu'tri tiv) ratio : the pro- 

 portion of protein in the diet. 



Oils : a class of nutrients composed of 



much carbon and hydrogen, with a 



little oxygen. 

 Ommatidium (om'd tld'I um) : one 



of the elements of a compound 



eye. 

 Operculum (6 pur'kti \um) : a lid or 



flap covering the gills of fishes. 

 Opsonin (Sp'so nin) : a substance in 



