_Megacephalic._ Large-headed. _Melt._ To change from the solid to the liquid form. _Mendel’s Law._ A principle formulated by Gregor Mendel with regard to the inheritance of characteristics. Fenton, following Walter, states it thus: “When two animals or plants unlike with respect to any character are crossed, the offspring of the first generation will be apparently like one of the parents in regard to the character in question. The parent which impresses its trait upon the offspring in this manner is called the _dominant_, while the one that fails to be visibly represented is the _recessive._ When, however, the hybrid progeny of this generation are in turn crossed with each other, they will produce a mixed lot of offspring, one-fourth of which will be like the dominant grandparent, one-fourth like the recessive one, and the remaining half like the parents which resembled the dominant grandparent, yet failed to breed true to it.” _Mercury._ Of the metallic elements, the only one which is ordinarily found in the liquid state. Quicksilver. Formula: Hg. _Mesozoic._ Relating to the geological period between the Paleozoic and the Cenozoic. _Metabolism._ The complete process of building up and breaking down protoplasm in a living organism. The process of digesting and storing nutritive matter and of excreting waste matter. _Metal._ An element which replaces or is capable of replacing the hydrogen of an acid. One of a class of substances typically ductile, malleable, lustrous, translucent only through very thin layers, conducting heat and electricity well. _Metaphysics._ The branch of philosophy dealing with knowing and being. This must disappear entirely as science advances. _Metazoa._ All the complex animals. All the animals except the Protozoa. _Metric system._ The decimal system of weights and measures generally used in many countries, employed for many purposes by scientists everywhere. A meter is about 1.094 yards. A kilometer is about .6214 of a mile. A cubic centimeter is about .061 of an inch. A kilogram is about 2.204 pounds. The C. G. S. (centimeter-gram-second) system is used in practically every science which deals with space, mass, and time. _Mho._ A unit of electrical conductivity. _Microbe._ A very small organism; in popular use, a disease-producing bacterium, a germ. _Micro-._ Small, in many compounds. _Mineral._ Pertaining to substances which are not organic. An ore. _Mineralogy._ The science of ores. _Miocene._ Relating to the middle division of the Tertiary (geological) period. _Mitosis._ Cell division and multiplication by the usual process. _Mixture._ A mass of more than one sort of material which is not chemically compounded or united. _Molecule._ The smallest amount of a substance which can exist with the properties belonging to the substance. _Molybdenum._ A metallic element. Symbol: Mo. _Momentum._ The quantity of motion of a moving body. It is equal to the mass multiplied by the velocity.