604 EVOLUTION, GENETICS, AND EUGENICS 



Fertilization. — The union of male and female gametes and the conse- 

 quent initiation of development of a new individual. 



Floras. — Groups of plants inhabiting a given geographic region or 

 geologic period. 



Gamete. — A mature male or female reproductive cell, containing the 

 haploid number of chromosomes; an egg or a spermatozoon. 



Gametic Reproduction. — See Sexual reproduction. 



Gastrula. — A stage in the development of metazoan animals in which 

 the embryo consists of two germ layers, ectoderm and endoderm. 



Gemmules. — Hypothetical inheritance units involved in Darwin's pro- 

 visional theory of pangenesis. 



Gene. — See Factor. 



Genetics. — The science which seeks to explain the resemblances and 

 differences in organism related by descent; the modern analytic and experi- 

 mental study of variation, heredity, and sex. 



Genotype. — A group of individuals all of which are alike in their genes 

 or factors. Contrast with Phenotype. 



Genotypic. — Pertaining to the germinal or hereditary constitution of an 

 organism. Contrast with Phenotypic. 



Genus. — An arbitrary group in the systematic classification of animals 

 or plants, ranking above the species and containing one or more species 

 possessing structural characters differing from those of other genera. 



Germ cell. — A cell specialized for sexual reproduction. Matured germ 

 cells are known as gametes. 



Germ plasm. — That part of the cell protoplasm which is believed to be 

 the material basis of heredity and is transferred from one generation to the 

 next. Contrast with Somotoplasm or Soma. 



Germinal continuity. — The concept of an unbroken stream of germ plasm 

 ffom generation to generation back to the beginning of life. 



Gonads. — The organs (ovaries or testes) that contain the reproductive 

 cells or germ cells, and sometimes also contain glandular tissue that func- 

 tions in the differentiation of secondary sexual characters. 



Gynandromorph. — An animal in which one part exhibits male characters 

 and another part female characters. 



Habitat. — The complex of environmental factors making up the sur- 

 roundings of any species or race. 



Haploid. — The reduced (one-half) number of chromosomes present only 

 in gametes. See Diploid. 



Hermaphrodite. — An individual organism possessing both ovaries and 

 testes. 



Heterogenesis. — See Mutation. 



Heterozygote. — An individual or a zygote resulting from the union of 

 unlike gametes. 



