iris (eye'ris) : diaphragm which surrounds the pu- 

 pil of the eye in vertebrates; colored portion 

 of the eye. 



iron lung: apparatus for producing breathing 

 movements in paralyzed people. 



irradiation (ir-ray-dee-ay'shun) of food: exposing 

 food to ultraviolet light in order to produce 

 vitamin D in it. 



irritability: ability of protoplasm to respond to 

 stimuli. 



islands of Langerhans: small groups of cells con- 

 stituting a ductless gland in the pancreas 

 that secrete insulin into the blood. 



isotopes (ice'oh-topes) : any of two or more forms 

 of a chemical element which differ in the 

 number of neutrons in the atom of that ele- 

 ment. Some isotopes are radioactive, such as 

 radioactive iodine or phosphorus. 



lacteal (lack'tee-al) : tiny lymphatic in the villus; 

 absorbs digested fats. 



larva: the form which hatches from the insect egg 

 in complete metamorphosis; also in some ani- 

 mals the young when it is very different from 

 the parent. 



larynx (lar'inks) : uppermost portion of the wind- 

 pipe which contains the vocal chords; also 

 called voice box. 



layering: form of vegetative reproduction in which 

 a twig still attached to the main plant touches 

 the ground and takes root. 



learning: changing of behavior as a direct or in- 

 direct response to the environment; also the 

 acquiring of knowledge. 



legumes (leg'youms) : pod-bearing plants making 

 up a large family; some used as food, feed, 

 or for improving the soil. 



lens (of the eye) : a part lying near the front of 

 the eyeball in the vertebrate eye that focuses 

 the light rays on the sensitive cells of the 

 retina. 



lenticel (len'ti-sel): opening through the bark of 

 dicot stems; permits passage of gases into 

 and out of the stem. 



leucocyte (lew'ko-site) : another name for white 

 blood cells of which there are several kinds. 



lichens (lie'kens) : "compound" plants consisting 

 of an alga (or in some a species of bacteria) 

 and a fungus living together. 



life activities: activities carried on by all living 

 things, such as food getting or food manu- 

 facture, digestion, assimilation, respira- 

 tion, excretion, irritability, and reproduc- 

 tion. 



ligament: band of fibrous tissue that connects 

 bones. 



linkage: condition in which two characters are not 

 assorted independently of one another be- 

 cause the determining genes lie in the same 

 chromosome and therefore remain together 

 in reduction division. 



lipases (lie'paces) : fat-digesting enzymes. 



lymph: liquid which surrounds the tissue cells in 



vertebrates; it is largely blood plasma which 

 has diffused from the capillaries. 



lymph node or gland: one of the glandlike struc- 

 tures occurring in many places along a lym- 

 phatic; it filters out certain white blood cells 

 and makes new ones. 



lymphatic (lim-fat'ick) : tube which carries 

 lymph. 



lysin (lie'sin) : antibody which breaks up bac- 

 terial cells or other cells which have entered 

 the body. 



macrospore (mac'roh-spore) : large spore in cer- 

 tain ferns and seed plants; it grows into the 

 female prothallus or, in seed plants, into the 

 embryo sac. 



maggot: larval stage of flies. 



mammals: members of a class of vertebrates hav- 

 ing hair, a diaphragm, and milk glands used 

 in feeding the young. 



marsupials ( mar-soo'pee-els ) : "pouched" mam- 

 mals; in most cases the young are born in 

 an immature state and live in the pouch for 

 a long lime after birth. 



maturation (mat-you-ray'shun) : process by which 

 eggs and sperms ready for fertilization are 

 formed from primary sex cells; in this proc- 

 ess the number of chromosomes is reduced to 

 the haploid or half number. 



medulla oblongata (med-dull'a ob-long-gah'ta ) : 

 the hindmost part of the brain continuous 

 with the spinal cord; the center for heart- 

 beat, breathing, etc. 



meninges (men-in'jees) : three membranes cov- 

 ering the brain and cord of vertebrates. 



mesentery (mes-en-ter'ree) : one of several folds 

 of thin membrane attached to the wall of the 

 abdomen of a vertebrate; it holds the organs 

 in place. 



mesoderm: middle layer of cells which forms be- 

 tween ectoderm and endoderm in the em- 

 bryo of a many-celled animal. 



metabolic (met-ah-bol'ic) disease: disease in 

 which there is abnormal building up or 

 breaking down of living matter in the animal 

 body. 



metabolism (met-ab'o-lism) : sum of all chemical 

 changes that go on in a cell or in the body 

 of an organism. Basal metabolism: amount 

 of metabolism when the body is as nearly 

 at rest as possible; a test for this is often 

 made to diagnose thyroid activity. 



metamorphic (met-e-mor'fic) rocks: rocks which 

 have been changed in structure by terrific 

 pressure or heat; marble, a metamorphic 

 rock, is changed limestone. 



metamorphosis (met-e-mor'fo-sis) : in zoology, 

 changes in an animal after its embryonic 

 stages by which it is adapted to a different 

 way of living, as the change from an insect 

 larva into the adult or a tadpole into a frog. 

 In incomplete metamorphosis in insects the 

 form hatching from the egg bears considera- 



591 



