FACTOR, the determiner of a particular hereditary character. FEEBLE-MINDEDNESS, deficiency in mental development. For grades, see p. 244. FERTILIZATION, union of the sexual cells. FETUS (L. _feuere_, to bring forth), the unborn young animal in its later (after the second month in man) stages of development. FLAGELLUM (L. _flagellum_, little whip), a vibratile, thread-like organ of locomotion. GAMETE (Gr. _gamos_, marriage), a mature germ-cell. GENETICS (Gr. _genesis_, origin), the science which deals with heredity and the origin of individuals in general. GENOTYPE (Gr. _genea_, race; _typto_, strike), the germinal constitution of an organism. GEOTROPISM (Gr. _ge_, earth; _trope_, turning), defined, p. 198. GERM-CELL, a reproductive cell. GERMINAL VARIATIONS, variations which owe their origin to some modification in the germ-cells. GERM-PLASM, the material basis of inheritance. GONAD (Gr. _gonos_, generation), a germ-gland. HAPLOID (Gr. _haploos_, single; _eidos_, form), the single or reduced number of chromosomes as found, for instance, in the mature germ-cells. HELIOTROPISM (Gr. _helios_, sun; _trope_, turning), defined, p. 198. HEREDITY (L. _heres_, heir), resemblance of individuals to their progenitors based on community of origin. HERITAGE (L. _heres_, heir), all that is inherited by an individual. HETEROZYGOTE (Gr. _heteros_, other; _zygon_, yolk), an individual produced through the union of germ-cells which are unlike in one or more determiners. Adjective, _heterozygous_. HOMOZYGOTE (Gr. _homos_, same; _zygon_, yolk), an individual produced through the union of germ-cells which are alike in determiners. Adjective, _homozygous_. HYBRID (L. _hybrida_, mongrel), the offspring of parents which differ in one or more characters. IDENTICAL TWINS, twins which show identical inborn characters, both having come presumably from the same ovum. IDIOT (Gr. _idios_, peculiar, private), defined, p. 244. IMBECILE (L. _imbecillis_, weak), defined, p. 244. INHERITANCE (L. _in_, in; _heres_, heir), the sum of all characters which are transmitted by the germ-cells from generation to generation. INHIBITOR (L. _in_, in; _habeo_, hold, have), that which checks or restrains. INSTINCT (L. _in_, in; _stingno_, prick), defined, p. 203. INTRA-UTERINE (L. _intra_, within; _uterus_, the womb), within the womb. IRRITABILITY (L. _irrito_, excite), the property of responding to stimuli. LININ (L. _linum_, flax), filaments of the cell-nucleus not readily stained by dyes. LUETIN TEST (L. _lues_, pest), a test for syphilis; see p. 188. MAMMALS (L. _mamma_, breast), warm-blooded, hairy animals which suckle their young. MATURATION (L. _maturus_, ripe), the final stages in the development of the sex-cells characterized by two divisions in one of which the number of chromosomes is reduced by one-half. MENDELIAN, MENDELISM, referring to Mendel, the founder of a theory of heredity. See p. 67.