fertilization: uniting two unlike gametes to form a 

 fertilized egg or zygote. 



fibrinogen (fye-brin'o-jen) : protein in blood plas- 

 ma which under certain conditions thickens 

 and forms fibers as part of the blood clot. 

 After hardening fibrinogen becomes fibrin. 



filament (fiU'a-ment) : slender stalk-like part of a 

 stamen; thread of an alga or fungus consist- 

 ing of a row of attached cells. 



filial (fill'i-al) generations: generations of off- 

 spring of a cross; first filial or Fj is the first 

 generation of offspring; second filial or F2 

 is the offspring of two individuals belonging 

 to the Fi generation. 



fishes: members of a class of vertebrates that have 

 gills, commonly fins, and a body usually cov- 

 ered with slimy scales. 



flagellum (fla-jell'um) ; pi. flagella: long whip-like 

 projection from a cell. 



food: any substance that can be used by a cell for 

 oxidation, secretion, or assimilation. 



food chain: series of organisms, always starting 

 with a green plant, with each organism serv- 

 ing as food for the one above it in the series. 



fossil: any remains, impression, or trace of an ani- 

 mal or plant of an earlier geological age; 

 most often found in rock. 



fraternal twins: developed from two distinct eggs, 

 each egg fertilized by a separate sperm; dis- 

 tinguished from identical twins. 



frond: leaf of a fern; also a leaflike part in sea- 

 weeds and other lower plants. 



fruit: ripened ovary together with any other at- 

 tached parts. 



function (funk'shun) : activity or useful action of 

 some organ, tissue, cell, or part of a cell. 



fungi (fun'jeye) : plants of the thallophyte phy- 

 lum which lack chlorophyll; for example, 

 mushrooms. 



gall (gawl) bladder or bile sac: sac attached to 

 the liver in vertebrates; holds bile secreted 

 by the liver. 



gamete (gam'eat) : either of the two cells that 

 unite in sexual reproduction to form a new 

 organism. The male gamete is the sperm; 

 the female is the egg or ovum. In conjuga- 

 tion, where the gametes are similar in struc- 

 ture, one may be an active or supplying gam- 

 ete, the other a passive or receiving gamete. 



ganglion (gan'glee-on) ; pi. ganglia: small group 

 of cell bodies of neurons. In vertebrates gan- 

 glia lie outside the brain and cord, some of 

 them being part of the autonomic system. In 

 invertebrates together with nerves they con- 

 stitute the nervous system. 



gastric juice: digestive juice of the stomach se- 

 creted by gastric glands. 



gastrula (gast'roo-la) : cuplike stage of the em- 

 bryo in many-celled animals; at first two lay- 

 ered, later three layered. 



gene (jean) : unit of heredity; substance located 

 in the chromosome and transmitted with it ; 



acting with the environment and other genes 

 it produces one or more characters in the or- 

 ganism. 



genus (jee'nus) ; pi. genera: in classification the 

 usual major subdivision of a family, consist- 

 ing of one or more species. The name of the 

 genus appears first in the scientific name of 

 an organism. 



geology (jee-ol'oh-jee) : the study of the earth, 

 the rocks of which it is composed, and the 

 changes it is undergoing or has undergone. 



geotropism (jee-ot'ro-pism) : tropism in which 

 the stimulus is gravity. 



germ: microscopic organism that causes disease; 

 also the beginnings of an organism. 



germ cells: reproductive or sex cells of an animal 

 or plant. 



germ layers (primary) : the three layers of cells 

 in the cup stage of the development of many- 

 celled animals; from each layer are formed 

 particular organs and tissues. 



germ plasm: protoplasm of the reproductive cells 

 and of those cells in the early stages of de- 

 velopment which will become reproductive 

 cells. 



germination (jer-min-ay'shun) : sprouting of a 

 seed or spore. 



gestation (jes-tay'shun) : in mammals the period 

 of development of the embryo within the 

 body of the parent. 



gills: breathing organs of aquatic animals, suited 

 to taking dissolved oxygen out of water; in 

 fish they consist of the arch which holds the 

 filaments containing capillaries and of rakers 

 which protect by straining the water taken 

 through the mouth. 



gill slits: in fish the openings back of the head 

 through which water leaves after it has 

 passed over the gill filaments; also found in 

 vertebrate embryos. 



gland: any cell or group of cells that secretes a 

 substance; the secreting cells are epithelial. 

 There are ductless glands and glands with 

 ducts. 



glucose (gloo'cose) : grape sugar; a simple sugar 

 with the formula CeHioOe- 



glycogen (gly'ko-jen) : "animal starch"; carbohy- 

 drate stored in the liver; found also in 

 muscles. 



goiter: enlarged thyroid gland which may be ac- 

 companied by an undersecretion of thyroxin 

 (endemic goiter) or an oversecretion (exoph- 

 thalmic goiter ) . 



grafting: transplanting living tissue in an animal 

 or plant from one organism to another; com- 

 monly done in plants to maintain or produce 

 more of a desired type. 



guano (gwa'no) : chiefly the solid wastes of sea 

 birds found in large deposits especially on 

 islands off the coast of Peru and in Chile. 



guard cells: two cells on either side of a leaf 

 stoma, which regulate size of the stoma. 



589 



