598 EVOLUTION, GENETICS, AND EUGENICS 



Chromatin. — The substance of which chromosomes are largely com- 

 posed, the chemical composition of which is not exactly known; supposed 

 to be the hereditary substance; so called because it stains readily with certain 

 dyes and can be easily seen under the microscope. 



Chromosomes. — The definite masses of chromatin that have a char- 

 acteristic number, size, and shape in any given species. 



Combinations. — New organic conditions due to a new assortment of old 

 factors already present in the germ plasm of parents. 



Commensalism. — The habitual living together of two or more different 

 species of animals or plants, involving more or less interdependence. 



Crossing-over. — Exchange of genes (factors) between homologous chro- 

 mosomes, believed to occur during synapsis. 



Cytology. — The detailed microscopic study of the structure of cells, 

 especially of germ cells. 



Cytoplasm. — All of the living material of the cell outside of the nucleus. 



Determiner. — See Gene or Factor. 



Dihybrid cross. — A crossing of two parents diflfering from each other 

 in only two pairs of allelomorphs. 



Diploid. — The maximum or full (duplex) number of chromosomes, found 

 in body cells and in the unmaturated germ cells; twice the gametic or 

 haploid number. 



Dominant. — A term applied to that member of an allelomorphic pair of 

 characters that has the capacity of manifesting an effect wholly or partly 

 to the exclusion of the effect of the other member (the recessive). 



Duplicate factors (genes). — Two or more factors located in different 

 chromosomes, either of which produces the same result. 



Egg. — The female germ cell; more precisely, the female gamete. 



Environment. — The sum of all influences upon the organism that have 

 their origin outside of the body. 



Epigenesis. — The doctrine that the germ cell is absolutely or relatively 

 structureless and that differentiation arises de novo through the interaction 

 of the protoplasm and the environment. Contrast with Preformation. 



Fi generation. — The first hybrid generation of a hybrid cross; exhibits 

 only the dominant characters of the two parents. 



Fa generation. — The offspring resulting from the interbreeding o\ indi- 

 viduals of the F, generation. 



Fj generation. — The offspring resulting from the interbreeding of indi- 

 viduals of the Fj generation. 



Factor. — A unit of inheritance situated at some particular locus of a 

 particular chromosome and transmitted according to the laws of Mendel, 

 a determiner, or gene. 



Faunas. — Groups of animals inhabiting a given geographic region ur 

 geologic period. 



