606 EVOLUTION, GENETICS, AND EUGENICS 



of spindle, chromosomes, etc., and resulting in longitudinal splitting of each 

 chromosome. 



Modification. — A change in the body or soma of an individual due to 

 functioning or to use and disuse; same as Acquired character. 



Monohybrid cross. — A cross between two individuals differing in only 

 one pair of allelomorphs. 



Morphological. — Structural; opposed to physiological. 



Multiple allelomorphs. — Different factors or genes occupying the same 

 locus of homologous chromosomes; the character conditioned by such factors. 



Multiple factors. — Two or more factors, usually alike in character, which 

 unite in various numbers to produce different quantitative expressions of 

 one character. Same as Duplicate factors. 



Mutant. — An individual having a different genotypic constitution from 

 its parents, as the result of a change in the germ plasm, and not as the result 

 of segregation or crossing-over. Some authorities include the type of 

 changes due to chromosomal aberrations under the head of mutants, while 

 others limit mutants to those resulting from factor changes. 



Mutation. — The germinal change resulting in the production of a 

 mutant. The definition of mutation differs according to differences of 

 opinion as to what should be included under the definition of a mutant. 

 Contrast with Induction. 



Neo-Darwinian. — A descriptive term applied to theories based on 

 Darwin's theory of natural selection. 



Neo-Lamarckian. — A descriptive term applied to theories based on 

 Lamarck's theory of the inheritance of acquired characters. 



Neo-mutationist. — A modern adherent of the mutation theory; especial- 

 ly one who is interested chiefly in tracing the origin of germinal changes that 

 are responsible for the appearance of mutants. 



Non-disjunction. — The failure of the two members of a pair of homolo- 

 gous chromosomes to separate in cell division, with the result that they both 

 pass into one daughter-cell, making one too many chromosomes in one 

 gamete and one too few in the other. 



Nucleus. — The body within a cell that contains the chromatin. 



Ontogenetic. — Pertaining to the life history of the individual. Con- 

 trast with Phylogenetic. 



Ontogeny. — The developmental history of the individual; same as 

 development or somatogenesis. Contrast with Phylogeny. 



Orthogenesis. — The apparent fact that races or groups of animals or 

 plants vary progressively along definite fines, each variation forming the 

 threshold of departure fur the next and the whole series forming a definite 

 sequence; definitely directed evolution. 



Ovule. — The body that contains the egg of flowering plants and becomes 

 the seed after fertilization and maturation. 



Ovum {pi. ova). — The egg; the type of gamete produced by a female. 



Pangenesis. — A provisional theory as to the mechanism of hereditary 



