Glossary - 691 



Meristem. Embryonic tissue of plants, from which 

 all the other tissues are derived by growth, cell 

 division, and differentiation. 



Mesentery. A thin sheet of tissue connecting an 

 organ, especially the enteron, with the body wall. 



Mesoderm. The cell layer between ectoderm and 

 endoderm in the embryos of all animals above the 

 coelenterates. The tissues derived from this laver. 



Mesocloea. A layer of intercellular material be- 

 tween ectoderm and endoderm in coelenterates. 



Metabolism. The sum total of chemical reactions 

 occurring in living cells and organisms. 



Metamorphosis. Transformation of a larva into the 

 adult form. 



Metaphase. The stage of mitosis or meiosis, in 

 which the chromosomes lie in the equatorial plane 

 of the spindle. 



Metazoa. Multicellular animals. 



Microbe. A microscopic organism. 



Microgamete. The smaller of the two kinds of 

 gametes, in organisms which have gametes of two 

 sizes. 



Microscopic. Invisible with the naked eye, but vis- 

 ible with the microscope; i.e., between about 0.1,a 

 and lOO^t in diameter. 



Microsporangium. A sporangium bearing micro- 

 spores. 



Microspore. The smaller of the two kinds of spores 

 (in plants which have spores of two sizes) , which 

 develops into a male gametophyte. 



Microsporophvll. A sporophyl bearing microspo- 

 rangia. 



Mitochondria. Numerous cytoplasmic organelles 

 having a characteristic (Fig. 2-13) internal struc- 

 ture as seen in elcctronmicrographs of cells gen- 

 erally; each mitochondrion represents an organized 

 team of metabolic enzymes. 



Mitosis. The usual process of cell division in most 

 organisms, involving a replication of each chromo- 

 some and the distribution of the daughter chromo- 

 somes to the daughter cells. 



Mixture. Any material composed of more than one 

 kind of molecule, (cf. Substance.) 



Modification. Any variation caused by nongenetic 

 factors. 



Molecules. The smallest particles into which any 

 substance can be divided without chemical change. 



Monoecious. Having the two kinds of gametes pro- 

 duced by the same individual; i.e., not having sepa- 

 rate sexes, (cf. Dioecious.) 



Monosaccharide. A simple sugar; i.e., a carbohy- 

 drate that cannot be decomposed by hydrolysis 

 into simpler carbohydrates. 



Morphogenesis. The production of the character- 

 istic form and structure of a cell or an organism. 



Motor neuron. A neuron that transmits excitation 

 directly to an effector. 



Mucus. A slimy fluid secreted by gland cells present 

 in many epithelia, called mucous membranes. 



Muscle. A tissue specialized for the production of 

 movement by contraction (i.e., shortening) of its 

 cells or fibers. A sheet, bundle, or mass of such 

 tissue. 



Mutation. Change in a gene into another gene. 



Myelin sheath. A "fatty" envelope surrounding a 



nerve fiber. 

 Myofibrils. Longitudinal fibrils in muscle cells. 

 Myonemes. Contractile fibrils in protozoans. 



Natural selection. The automatic selection of or- 

 ganisms for superior viability and fertility under 

 natural conditions, resulting from the tendency of 

 all organisms to increase in numbers by reproduc- 

 tion and the limitation of this increase by environ- 

 mental factors. 



Nephridia. Excretory organs found in many inverte- 

 brates. 



Nerve. A group or bundle of nerve fibers. 



Nerve cells. A cell specialized for the transmission, 

 generation, or modification of excitation. 



Nerve fibers. A filamentous cytoplasmic extension 

 of a nerve cell. 



Nerve impulse. A state of excitation transmitted 

 along a nerve fiber. 



Neuroid transmission. The transmission of excita- 

 tion by cells other than nerve cells. 



Neuron. A nerve cell. 



Nitrifying bacteria. Bacteria that oxidize am- 

 monia to nitrites, or nitrite to nitrates. 



Nitrogen-fixing bacteria. Bacteria that form ni- 

 trate (-XO,) compounds from free nitrogen. 



Notochord. A longitudinal solid rod above the en- 

 teron in all chordates, at least in embryonic stages; 

 replaced later, in the vertebrates, by the vertebral 

 column. 



Nucleolus. One or more Feulgen-negative, intra- 

 nuclear bodies, concerned with the synthesis of 

 RNA. 



Nucleoproteins. Conjugated proteins formed by 

 union between protein and nucleic acid; of two 

 principal kinds: (1) deoxyribonucleoprotein (DNA 

 protein) , which is closely identified with the genie 

 material, and (2) ribonucleoprotein (RNA pro- 

 tein) , varieties of which are concerned with protein 

 synthesis. 



Nucleotides. Sugar-phosphate-base (purine and 

 pyrimidine) units that become bonded into very 

 long molecules, the nucleic acids, by a complex 

 process of polymerization. 



Nucleus. The clearly delimited portion of a cell 

 containing the chromatin. 



Nutrition. The intake of food substances, their dis- 

 tribution within the organism, metabolism, and 

 elimination of waste products. 



Oocytes. The auxocytes of the female, which give 



rise to the egg cells. 

 Oogonia. The gonia of the female, which finally 



give rise to oocytes. 

 Order. A taxonomic group, between class and fam- 



ily. 

 Organ. A localized part of an organism, specialized 



for some particular function. 

 Organic acids. Organic compounds, usually posses- 



ing a carboxyl radical (COOH) . 

 Organic bases. Organic compounds derived from 



ammonia (NH,) by the replacement of one or more 



of its hydrogen atoms by organic radicals. Organic 



