ble reKemblanre to the adult ; in complete 

 metamorphosis there is very little resem- 

 blance. 



microorganism (my-cro-or'gan-ism) : organism 

 whose structure can be seen only with the 

 aid of a microscope. 



mycropyle (my'cro-pile) : tiny opening in the 

 coats of an ovule (and later of the seed) ; 

 pollen tubes can enter through it. 



microspore: small spore in certain ferns and in 

 seed plants; in ferns it develops into the male 

 prothallus and in seed plants into the pollen 

 grain. 



mitosis (mit-toe'sis) : complicated nuclear divi- 

 sion occurring in normal cell division in all 

 animals and plants, except possibly some of 

 the simplest ; in mitosis each chromosome 

 and gene divides longitudinally in half. 



molars: grinding teeth of mammals. 



molds: any of the fungi that produce a downy or 

 furry growth on vegetable or animal matter. 



molecule (moll'e-kewl ) : smallest particle of a 

 substance that has the characteristics of the 

 substance; it may consist of one atom or 

 many thousands of atoms. 



mollusks (mol'lusks) : invertebrates with a soft 

 unsegmented body, with gills, mantle, and 

 foot, and usually covered by a shell of lime; 

 a large phylum, including clams, snails, and 

 octopuses. 



one of the three main stocks of liv- 

 mg men; it includes, among others, Mongo- 

 lians and American Indians. 



monocotyledons (mono-cot-i-lee'donsi or mono- 

 cots: members of a subclass of flowering 

 plants which have seeds with one cotyledon, 

 parallel-veined leaves, and scattered bundles 

 of tubes in the stem. 



mosses: plants of many species belonging to the 

 bryophyte group, with simple leaflike, root- 

 like, and stemlike parts; mosses have an al- 

 ternation of generations in their life cycle. 



mucous (mew'kus) membrane: lubricating mem- 

 brane that lines the alimentary canal and 

 some other organs in higher animals. 



mucus: slimy substance secreted by mucous mem- 

 brane. 



muscle tissue: animal tissue consisting of con- 

 tractile fibers. Striated or voluntary muscle: 

 muscle attached to parts of the skeleton; 

 smooth or involuntary muscle: muscle found 

 in internal organs. Cardiac muscle: muscle 

 found in the heart. 



mutant (mew'tant) : plant or animal that shows 

 a mutation. 



mutation (mew-tay'shun) : a change in a gene in 

 the germ plasm which will at some time ap- 

 pear in the oflspring, such as the character 

 of white eyes among red-eyed Drosophila. 



narcotic: any of a group of substances that blunt 

 the senses. 



natural selection, Darwin's theory of: theory to 



Mongoloid 



explain how new types of living things ap- 

 pear on the earth. (Darwin believed that 

 through the operation of natural causes the 

 fittest, in general, survive to become the par- 

 ents of the next generation, thus leading to 

 changes in types.) 



nectar: sweet liquid secreted by many flowers 

 which attracts some insects and birds; bees 

 can make honey from it. 



Negroid: one of the three main stocks of living 

 men; it includes, among others, the Negro 

 and Melanesian races. 



nerve: a bundle of nerve fibers or axons arising 

 in the brain, spinal cord, or ganglia and end- 

 ing among other cells of the body. Each fiber 

 in a nerve is a projection from the cell body 

 of a nerve cell. 



neuron (new'ron) : a nerve cell; afferent or sen- 

 sory neurons send the impulse toward the 

 brain or cord; efferent or motor neurons 

 carry the impulse away from brain or cord; 

 intermediary neurons lie between the afferent 

 and efferent. 



New Stone or Neolithic Age: period in the his- 

 tory of civilization which followed the Old 

 Stone Age. It ended about 6,000 years ago. 



niacin (nye'a-sin) : vitamin of the B complex 

 group; niacin deficiency may cause pellagra. 



nicotine: strong poison found in the tobacco 

 plant. 



nitrate (nye'traytl: compound containing nitro- 

 gen, oxygen, and at least one other element; 

 used by plants in making proteins. 



nitrifying (nye'tri-fy-ing) bacteria: bacteria in the 

 soil that build up ammonium compounds into 

 nitrates. 



nitrogen cycle: passage of nitrogen atoms from 

 the element as found in the air through more 

 and more complex compounds into living 

 matter and back again to the free nitrogen 

 of the air. 



nitrogen-fixing bacteria: bacteria that build up 

 proteins from free nitrogen; some kinds are 

 found in nodules on the roots of legumes; 

 other kinds live free in the soil. 



nodule (nod'youl) : small rounded mass, as on the 

 roots of legumes in which live nitrogen-fixing 

 bacteria. 



nuclear membrane (new'klee-er) : thin living 

 layer around the nucleus. 



nucleolus (new-klee'o-lus) : rounded body within 

 the nucleus that takes stain like chromatin 

 but is different from chromatin. 



nucleus (new'klee-us) : small ball of denser pro- 

 toplasm, lying within the cytoplasm, contain- 

 ing chromatin material. 



oesophagus (ee-sof'a-gus) : food pipe or gullet in 

 many animals; in man tube connecting the 

 throat with the stomach. 



Old Stone or Paleolithic (pay-lee-oh-lith'ic) Age: 

 first period in the history of civilization; 

 ended about 10,000 years ago. 



592 



