GENERAL ZOOLOGY 



GAMETOCYTE. A cell capable of differentiating into a gamete. 



GAMETOGENESIS. The differentiation of gametes. Cf. HISTOGENESIS. 



GANGLION. A mass of nerve cell bodies. 



GAS EXCHANGE. The exchange of respiratory gases (oxygen and carbon dioxide) 

 between the animal and its environment, either generally over the surface of 

 the bodv or in specialized organs such as gills or lungs. 



GASTRIC. Pertaining to the stomach. 



GASTRULA. .\ developing individual characterized by a diploblastic structure. 



GASTRULA CAVITY. The cavity of the gastrula, lined with endoderm and opening 

 b\ \\a\- of the blastopore; the archenteron or gastrocoel. 



GASTRULATION. The formation of a gastrula from a blastula by cell localization. 



GEL. A semisolid or jelly-like state of protoplasm or any colloid. CJ. SOL. 



GEMMULE. A reproductive body resistant to unfavorable environmental conditions, 

 capable of germinating to produce a new individual; characteristic of many 

 sponges. The statoblasts of fresh-water Ectoprocta are analogous formations. 



GENE COMPLEX. .Ml the genes present in a zygote. 



GENE STRING. .\ linear series of genes constituting a linkage group. 



GENERIC. Pertaining to a genus. 



GENES. The units of inheritance, transmitted from one generation to the next by 

 way of the gametes and conditioning the appearance of an individual's char- 

 acters during its development. 



GENETICIST. A student of heredity and variation; more specifically, a .student of 

 experimental breeding. 



GENETICS. The science of heredity and variation. 



GENITAL. Pertaining to the organs of reproduction. 



GENONEMATA {sim;^. GENONEMA). Slender threads of chromatin which can be dis- 

 tinguished within the chromosomes during mitosis, and sometimes in the nucleus 

 l>ctween periods of division. 



GENUS {pi. GENERA). One of the subdivisions of a family; a group of next higher 

 rank than a species. 



GERM CELL. A cell, in a multicellular organism, that is capable of reproduction by 

 s\ ngamN . (,/ SOMATIC CELL and GAMETE. 



GERM LAYERS. The cell layers resulting from the early cell localizations during the 

 establishment of the body plan in development; specificalh', ectoderm, endo- 

 derm, and mesoderm. Cf. DIPLOBLASTIC and TRIPLOBLASTIC. 



GILL. An organ of external respiratory exchange in branchiate aciuatic animals. 



GILL SLIT. /\ pharyngeal cleft; one of the paired openings that appear in the region 

 of the pharynx during the development of hemichords and chordates, and along 

 the sides of which gills develop in the aquatic chordates. 



GLAND. One or more cells differentiated for the production of some secretion. 



GONAD. An organ in which gametes are differentiated; an ovary or testis. 



GONODUCT. A duct through which gametes or embryos pa.ss out of an animal. 



GONOPORE. External opening of a reproductive duct. 



GROWTH. An increase in the size of an organism that results when anabolic processes 

 occur at a more rapid rate than catabolic proce.s.ses. 



GROWTH PERIOD. 'I'he period of gametogenesis that immediately precedes the 

 meiotic divisions. 



GUSTATORY. Pertaining to lasting. 



GUT. The digestive tract. 



HABITAT. The area or region in which an organism li\es; the environmental con- 

 ditions under which a species can exist. 



HAPLOID. Having a single set of chromosomes that do not occur in pairs. Gametes 

 are haploid cells. Cf. DIPLOID. 



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