METAZOA (Gr. _meta_, over; _zoon_, animal), all animals higher than the protozoa. MITOSIS (Gr. _mitos_, thread), indirect nuclear division, characterized by the appearance of a fibrous spindle and a definite number of chromosomes. The latter split to form daughter chromosomes which diverge to the poles of the spindle to form parts of the new nuclei. MONGOLIAN, a type of feeble-minded individual, see p. 248. MONOHYBRID (Gr. _monos_, single; L. _hybrida_, mongrel), the offspring of parents, differing in one character. MORON (Gr. _moros_, foolish), defined, p. 244. MUTATIONS (L. _mutare_, to change), abrupt, inheritable germinal variations. Frequently though not necessarily they are changes of considerable extent. NEURAL (Gr. _neuron_, nerve), pertaining to the nervous system. NEURON (Gr. _neuron_, nerve), a nerve-unit consisting of a nerve-cell with branching processes called dendrites and an axon or axis cylinder process which gives rise to a nerve fiber. NEUROPATHIC (Gr. _neuron_, nerve; _pathos_, suffering), relating to disease of the nervous system. NUCLEOLUS (L. dim. of nucleus), a well-defined body found within the nucleus of a cell. NUCLEUS (L. _nux_, a nut), the central organ of a cell. NULLIPLEX (L. _nullus_, not any; _plico_, fold), the condition in which no determiners of a given character exist in a particular individual. OÖCYTE (Gr. _oon_, egg; _kytos_, cell), the ovarian egg in one stage of development. OÖGENESIS (Gr. _oon_, egg; _genesis_, origin), the development of ova from primitive sex-cells. OÖGONIUM (Gr. _oon_, egg; _gonos_, generation), a primordial egg-cell. OVARY (L. _ovum_, egg), the organ in which the egg-cells multiply and are nourished. OVUM (L. _ovum_, an egg), the female sex cell. PARTHENOGENESIS (Gr. _parthenos_, virgin; _genesis_, origin), development of an egg which has not united with a male gamete. PHENOTYPE (Gr. _phaino_, show; _typto_, strike), the existing type of individual irrespective of hereditary possibilities which may reside in it undeveloped. PHOTOTROPISM (Gr. _phos_, light; _trope_, turning), defined, p. 198. PLACENTA (L. _placenta_, a flat cake), the organ by which the fetus of the higher mammals is attached to the uterine wall of the mother for purposes of nourishment, respiration and excretion. In it the maternal and fetal blood, although not intermingling, are brought into such close proximity that an interchange of dissolved substances is possible. POLAR BODIES, the minute cells which are separated from the egg in its maturation divisions. PRIMATE (L. _primus_, first), the highest order of animals, including monkeys, apes and man. PRONUCLEUS, the nucleus of the mature ovum or sperm-cell. PROTOPLASM (Gr. _protos_, first; _plasma_, form), the essential living substance. PROTOZOA (Gr. _protos_, first; _zoon_, animal), single-celled animals or animals composed of cells not separable into different tissues. PSYCHICAL (Gr. _psyche_, the soul), pertaining to the mind.