_Litopterna_: A suborder of extinct ungulate mammals from the Miocene and Pliocene of South America resembling horses or llamas. _Mammals_: Vertebrate animals which suckle their young after birth. _Meiosis_: The process whereby the chromosomes of synaptic pairs (in the primary oöcyte or spermatocyte) are separated in such a way that the resulting gametes (eggs, or sperms) receive a haploid (halved) number of unpaired chromosomes, instead of the diploid (double) number of paired chromosomes characteristic of the zygote and the somatic cells of the species. _Metista_: Animals and plants normally multicellular and having their cells differentiated into at least two distinct layers or tissues—the Metazoans and Metaphytes. _Mitosis_: Typical cell-division, whose mechanism consists of the spindle-fibers, and whose scope is to secure an exactly equal partition of the single components of the nucleus of the dividing cell between the two resultant daughter-cells. _Monism_: A system of thought which holds that there is but one substance, either mind (idealistic subjectivism), or matter (objectivistic materialism),—or else a substance that is neither mind nor matter, but is the substantial ground of both. Idealistic monism regards mind as the sole reality and matter as its product. Materialistic monism regards matter as the sole reality and mind as its product. _Neolithic_: Pertaining to the Young-Stone Age, that is, to prehistoric man of Post-glacial time. The implements of the latter are of polished stone. The Young-Stone Age is said to have begun about 7,000 years B.C., and to have ended with the Copper Culture about 2,000 B.C. The Bronze Age, which followed it, belongs to history. _Neurone_: The nerve-cell with all its processes, consisting, therefore, of the nucleated cell-body, the axone or discharging fiber, and the dentrites or receiving fibers. _Oölites_: An English term for the Jurassic, or middle system of the Mesozoic group of fossiliferous rocks. _Ontogeny_: The embryological development of the individual. _Opposable_: A term applied to the thumb or great toe when they are capable of being placed with their tips opposite to those of the other digits. _Organelle_: Literally, a “miniature organ,” _i.e._, one of the living components of a cell as distinguished from the metaplastic or non-living inclusions. _Oxychromatin_: That portion of the nuclear network which stains with acidic dyes, the finer nuclear reticulum in which the coarser strands of basichromatin appear to be suspended.