388 PRINCIPLES OF ANIMAL BIOLOGY 



Marsupial. A mammal having a pouch in which the young are carried (for example, 



the opossum and the kangaroo). As an adjective, possessing a pouch; as the 



marsupial frog. 

 Mastigophora {mas' ti gof o ra). A class of protozoa, characterized by flagella. 

 Mastodon {mas' to don). An extinct genus of elephantlike animals of Pliocene and 



Pleistocene time. 

 Maternal. Pertaining to or derived from the mother. 

 Matrix. The noncellular material in which the cells of bone and cartilage are 



imbedded. 

 Matter. That of which any physical object is composed; anything which can occupy 



space. 

 Maturation. A process which germ cells undergo before they become functional, 



consisting of one or two cell divisions; if of two divisions, the chromosomes remain 



imduplicated in one of them. 

 Medulla oblongata. The enlargement of the anterior end of the spinal cord in verte- 

 brates, commonly regarded as the posterior division of the brain. 

 Medusa (pZ., medusae). A jellyfish; the free-swimming member of many hydroid 



species. 

 Megapode {meg' a pode). A bird of the family Megapodiidae, the mound birds and 



jungle fowls. 

 Meiosis {mi o' sis). Separation of maternal from paternal chromosomes in oogenesis 



or spermatogenesis. Also, according to some, the two divisions of oogenesis or 



spermatogenesis. 

 Mendel, Gregor. Austrian monk and plant breeder, founder of modern movement 



in genetics, and author of Mendel's law of heredity. Lived 1822-1884. 

 Mendel's law. The law that genes for inherited characters separate from one 



another and recombine in various ways in the germ cells. 

 Meridional {me rid' i o nal). Passing through the animal and vegetative poles; said 



of certain cleavage planes of an egg. 

 Merychippus imer' i kip' pus). An extinct horselike animal of Miocene time. 

 Mesenchyme {mes' en kime). A tissue composed of cells of irregular shape, loosely 



joined in a network with extensive meshes. 

 Mesentery {mes' en ter i). A double sheet of tissue, continuous with the peritoneum, 



which supports an organ (such as the intestine) from the body wall. 

 Mesoderm. A layer of cells between the ectoderm and endoderm. 

 Mesohippus. An extinct animal of Oligocene time, ancestral to the horse. 

 Mesozoa. A group of degenerate animals of uncertain rank and relationship, onco 



regarded as intermediate between protozoa and metazoa; hence the name. 

 Mesozoic. Pertaining to the geological era between the Paleozoic and Cenozoic, 



or the age of reptiles. 

 Metabolism {me tab' o liz'tn). The sum total of the chemical processes going on in 



protoplasm. 

 Metacarpal. One of the bones forming the body of tli(^ liand or forefoot in vertolnates. 

 Metagenesis. The occurrence of two or more forms of individual in the same sj)ecies, 



one of which reproduces sexually and one or more asexually. 

 Metamere. See somite. 

 Metamerism {me tam' er iz'ni). The condition of being divided into a number of 



similar metameres or somites. 

 Metamorphosis {met' a mor' fo sis). The transformation of a larva into an adult. 

 Metaphase. That stage of cell division in which the chromosomes are in the equa- 

 torial plate. The chromosomes are typically (hii)li('ate(l in this stage. 

 Metatarsal. One of the bones forming the body of tlu' iliindi loot of vertebrates. 



