468 GENERAL ZOOLOGY 



External respiration. The passage of oxygen from the air or water into 



the blood. 

 Extracellular. Outside of cells. 



Factor. Any causative agent. Its effect is called response. 

 Family. A subdivision of an order in classification. 

 Fat. An organic compound consisting of carbon, hydrogen, and oxygen, 



in the form of a glyceric ester of a fatty acid. 

 Fauna. The animal organisms occurring in a given region or place. 

 Feces. The excrement or undigested food residue discharged from the 



alimentary canal. 

 Fermentation. The decomposition of organic compounds, usually 



through the action of enzymes {ferments). 

 Fertilization. The culmination of the series of events following the 



entrance of the sperm into an egg, ending in the union of a male and of 



a female nucleus to form the first cleavage nucleus. 

 Fetus. The mammalian embryo in later stages of development when the 



body regions are well defined. 

 Flagellum. A greatly enlarged cilium occurring in Protozoa, and fre- 

 quently as the tail of a spermatozoon. 

 Fluctuating variation. A relatively slight variation of somatic origin 



which is not heritable. 

 Fossil. The remains of a prehistoric organism or its tracks, etc., found 



usually embedded in the earth's crust. 

 Gamete. Male or female germ cell; sperm or egg. 

 Ganglion. A tissue mass composed principally of nerve-cell bodies. 

 Gastric. Pertaining to the stomach. 

 Gastrocoel. The cavity of the gastrula. 

 Gastrula. In embryogeny, the double-walled sac resulting from the 



invagination of the single-layered blastula. The outer layer is ecto- 

 derm and the inner endoderm. Gastrulation is a fundamental process 



in development. 

 Gel. A more or less rigid colloidal state. 

 Gene. The germinal representative of a character. 

 Genotype. The factor or gene complex of an organism or a group of 



organisms. The germinal complex as contrasted with somatic. 

 Genus. A group of related species. A subdivision of a family in 



classification. 

 Germ layer. One of the primary embryonic tissues, ectoderm, endoderm, 



or mesoderm, from which the tissues and the organs of the adult develop. 

 Gestation. The period of development in viviparous animals between 



fertilization and birth. 

 Gill. A platelike or filamentous appendage of an aquatic animal bathed 



by water and serving as an organ of respiration. 

 Gill clefts. Paired openings leading from the sides of the pharynx 



to the exterior. Present in all Chordates. In fishes gills are attached 



to the septa separating the clefts. 

 Gland. An organ whose cells secrete one or more substances that may 



be used by the organism (as in secretions of the glands of the alimentary 



