GLOSSARY 439 



GENE. A factor or element in the chromosomes of the germ cells 

 which conditions a character of an organism. 



GENETICS. The science of heredity. 



GENOTYPE. The fundamental hereditary constitution of an organ- 

 ism or group of organisms. The gene complex of an organism. 

 See Phenotype. 



GENUS. In classification, a main subdivision of a family. See 

 Species. 



GERMINAL CONTINUITY. The concept of an unbroken stream of 

 germ plasm from the beginning of life, from which each genera- 

 tion is derived. 



GERM LAYER. A primary tissue (ectoderm, endoderm, or meso- 

 derm) in the embryo from which the tissues and organs of the 

 adult animal develop. 



GERM LAYER THEORY. The doctrine that the germ layers are 

 fundamentally similar throughout the Metazoa and that homolo- 

 gous structures in various animals are derived during ontogeny 

 from the same germ layer. 



GERM PLASM. The physical basis of inheritance. The chromatin 

 which forms the specific bond of continuity between parent and 

 offspring. Contrasted with soma or somatoplasm. 



GILL SLITS. Paired lateral openings leading from the anterior end 

 of the alimentary canal to the exterior for the exit of the respira- 

 tory current of water. Permanent or embryonic characters 

 of Vertebrates. Branchial clefts. 



GLAND. One cell or a group of many epithelial cells which elaborate 

 certain materials and then secrete the product for the use of the 

 organism. 



GLOTTIS. The opening from the pharynx into the tube (trachea) 

 leading to the lungs. 



GONAD. An organ in which the germ cells develop. Ovary or 

 test is. 



GREEN GLANDS. Excretory organs (nephridia) of the Crayfish 

 and its allies. 



HAPLOID. The reduced (one-half) number (simplex group) of 

 chromosomes. See Diploid. 



