GLOSSARY 471 



Macroscopic. Large enough to be seen with the unaided eye. 



Mammal. A vertebrate having hair and mammary glands. 



Mandible. A biting mouth part of invertebrates. The lower jaw of 



vertebrates. 

 Maternal. Pertaining to or derived from the female parent. 

 Matrix. The intercellular material of cartilage and bone. Intercellular 



substance. 

 Maxilla. A mouth part of an invertebrate. The upper jaw of vertebrates. 

 Mechanism. The hypothesis supported by many facts that the phenomena 



of life are inherent in the physical and chemical properties of the con- 

 stituents of protoplasm. It does not admit "vital force" as a factor in 



explaining life processes. 

 Mendelism. A universal type of inheritance based on the fact that genes 



of inherited characters separate and combine as units in the germ cells. 

 Mesoderm. The embryonic germ layer formed between the endoderm and 



ectoderm. 

 Mesoglea. A noncellular layer lying between the ectoderm and endo- 

 derm of Hydra and related forms. 

 Metabolism. The chemical processes of protoplasm which are made up 



of disintegrative and reintegrative phases, catabolism and anabolism, 



respectively. 

 Metagenesis. The alternation of sexual and asexual generations in the 



life history of an organism. The alternation of sexual medusa and 



asexual polyp in Obelia. 

 Metamerism. The repetition of parts or segments in a linear series, as in 



the segmentation of the earthworm, or the arrangement of vertebrae 



in the vertebral column. 

 Metamorphosis. The more or less sudden change of the larva into the 



adult. The transformation of a tadpole into a frog; or of a caterpillar 



into a moth. 

 Metazoa. Animals whose bodies consist of more than one cell. All 



animals above Protozoa. 

 Millimeter. One-tenth of a centimeter. Abbreviation: mm. 

 Mitosis. The ordinary form of cell division. Also called karyokinesis. 

 Molecule. A group of atoms. The smallest particle of a substance that 



possesses the properties of the substance. 

 Monoecious. Having both ovary and testis in one individual. 

 Motor neuron and nerve. One carrying impulses away from the central 



nervous system and causing muscular contraction or glandular activity. 



Efferent neuron. 

 Mucosa. The layer of cells lining the digestive tract of vertebrates. 

 Mutation. A heritable variation of a discontinuous type caused by some 



sort of change in the germplasm. 

 Myotome. One of the segments in the body musculature of vertebrates. 

 Natural selection. The natural process of eliminating the unfit. The 



survival of the fittest in nature. 

 Nephridium. A tubular type of excretory organ such as occurs in the earth- 

 worm and which is the forerunner of the tubules of the vertebrate 



kidney. 



