|
Home >>Biology Dictionary >> Marsupium - Meiosis
|
|
Manus. Hand. Carpus, metacarpus, and digits of tetrapod forelimb
Marsh. An area of wet or periodically wet land where the soils are not peaty.
Marsupium. Pouch of many marsupials and Echidna. Fold of skin supported by epipubic bones of hip girdle, forming pouch containing mammary glands, into which the new born (or eggs of Echidna) are placed. Young are attached to teats in marsupials; lick milk in Echidna which has no teats.
Mass flow. Hypothesis of transport of materials in phloem. Sieve-tubes and contents are regarded as forming an osmotic system continuous throughout the plant connected, via plasmodesmata with other tissues.
Mastication. The process of chewing and breaking down food particles before swallowing.
Mastigonemes. Exceedingly fine lateral projections along the length of vertain flagella (‘tinsel flagella’). Thought to increase mechanical efficiency of flagellum by increasing surface area.
|
Material. The sample available for any study in Taxonomy.
Matter. That which constitutes the substance of physical forms, has mass, occupies space and can be quantified.
Matrix (in histology). The component of tissues (e.g. bone and cartilage) in which the cells of the tissue are embedded.
Maturation. Final stages in preparation of sex cells for mating, with segregation of homologous chromosomes so that each cell or gamete contains half the usual number.
|
Maxilla.
(1) One of a pair or two pairs of feeding appendages (mouthparts) of various arthropods, such as crustaceans, insects, millipedes and centipedes.
(2) One of a pair of large bones of the upper jaw of vertebrates. In mammals they bear the molar and premolar teeth.
Maze. A Labyrinth of winding passages used in behavioural experiments involving learning.
Meatus. Any natural opening or passage particularly the external auditory meatus leading to the Tympanum in the ear.
Mackel’s cartilage. A paired cartilage forming the lower jaw in cartilaginous fishes such as sharks, skates and dog fish. In bony fish (Osteichthyes), reptiles and birds, it is ossified to form the articular bone. In mammals it persists as an ear ossicle, the malleus.
Mecoptera. An order of insects containing the scorpion flies.
|
|
Medulla. The inner portion of an organ, as opposed to the cortex. Medulla is the posterior region of vertebrate brain. Of plants, pith ; central core of usually parenchymatous tissue.
Medulla oblongata. A region of the hind brain that is concerned with the functioning of the visceral organs, e.g. the stomach, lungs and heart.
Medullary plate. The neural plate of the vertebrate embryo.
Medullary ray. Parenchyma cells running radially through the vascular tissue of a stem or root. They may be primary or secondary and are called vascular rays (secondary) when cut off by the cambium.
|
|
|
Medusa. A free-swimming organism, sexual form of Zooid in coelenterate with gelatinous body of bell or umbrella shape, margined with tentacles and having the mouth centered on a projection of the concave surface. Bell or umbrella-shaped, swimming by pulsations of body. Generally do not reproduce by budding. Reproduce sexually, fertilized eggs giving rise to polyps and are themselves produced asexually by polyps.
Meganucleus. See Macronucleus.
Megasporangium. The organ where megaspores are formed which in flowering plants is the ovule
Meiosis (reduction division).
A type of cell division that gives rise to four reproductive cells (gametes) each with half the chromosome number of the parent cell. Two consecutive divisions occur (See illustration). In the first, homologous chromosomes become paired and may exchange genetic material before moving away from each other into separate daughter nuclei. This is the actual reduction division because each of the two nuclei so formed contains only half of the original chromosomes. The daughter nuclei then divided by mitosis and four haploid cells are produced.
|
Meiospore. A spore formed as the result of Meiosis.
Melanin. Any of a group of polymers, derived from the amino acid tyrosine, that cause pigmentation of eyes, skin, and haid in vertebrates. Melanins are produced by specialized epidermal cells called melanocytes. Certain invertebrates, fungi, and microorganisms also produce melanin pigments. The ‘ink’ of the octopus and squid is a notable example. Hereditary albinism is caused by the absence of the enzyme tyrosinase, which is necessary for melanin production.
|
|
