macroevolution
(L. makros, long, large, + evolvere, to unfold). Evolutionary change on a grand scale, encompassing the origin of
novel designs, evolutionary trends, adaptive
radiation, and mass extinction.
macrogamete
(Gr. makros,
long, large, + gamos, marriage). The larger
of the two gamete types in a heterogametic
organism, considered the female gamete.
macromere
(Gr. makros, long,
large, + meros, part). The largest size class of
blastomeres in a cleaving embryo when the
blastomeres differ in size from one another.
macromolecule
A very large molecule, such
as a protein, polysaccharide, or nucleic acid.
macronucleus
(Gr. makros, long, large, + nucleus, kernel). The
larger of the two kinds of nuclei in ciliate
protozoa; controls all cell functions except
reproduction.
macrophage
(Gr. makros, long,
large, + phago, to eat). A phagocytic cell
type in vertebrates that performs crucial
functions in the immune response and
inflammation, such as presenting antigenic
epitopes to T cells and producing several
cytokines.
madreporite
(Fr. madr + pore,
reef-building coral, + ite, suffix for some
body parts). Sievelike structure that is the
intake for the water-vascular system of
echinoderms.
major histocompatibility complex (MHC)
Complex of genes coding for proteins
inserted in the cell membrane; the proteins
are the basis of self-nonself recognition by
the immune system.
malacostracan
(Gr. malako, soft, + ostracon, shell). Any
member of the crustacean subclass
Malacostraca, which includes both aquatic
and terrestrial forms of crabs, lobsters,
shrimps, pillbugs, sand fleas, and others.
malleus
(L. hammer). The ossicle
attached to the tympanum in middle ears of
mammals.
malpighian tubules
(Marcello
Malpighi, Italian anatomist, 1628 - 1694).
Blind tubules opening into the hindgut of
nearly all insects and some myriapods and
arachnids, and functioning primarily as
excretory organs.
mantle
Soft extension of the body wall in
certain invertebrates, for example,
brachiopods and molluscs, which usually
secretes a shell; thin body wall of
tunicates.
manubrium
(L. handle). The
portion projecting from the oral side of a
jellyfish medusa, bearing the mouth; oral
cone; presternum or anterior part of
sternum; handle-like part of malleus of ear.
marasmus
(Gr. marasmos, to
waste away). Malnutrition, especially of
infants, caused by a diet deficient in both
calories and protein.
marsupial
(Gr. marsypion, little
pouch). One of the pouched mammals of
the subclass Metatheria.
mast cells
Inflammatory cells in a variety of
locations. Upon activation by an antigen
they release pharmacologically active
compounds leading to redness and swelling.
mastax
(Gr. jaws). Pharyngeal mill
of rotifers.
matrix
(ma´triks) (L. mater, mother). The
intercellular substance of a tissue, or that
part of a tissue into which an organ or
process is set.
maturation
(L. maturus, ripe). The process of
ripening; the final stages in the preparation
of gametes for fertilization.
maxilla
(L. dim. of mala, jaw).
One of the upper jawbones in vertebrates;
one of the head appendages in
arthropods.
maxilliped
(L. maxilla, jaw, + pes, foot). One of the pairs of head
appendages located just posterior to the
maxilla in crustaceans, a thoracic appendage
that has become incorporated into the
feeding mouthparts.
medial
Situated, or occurring, in
the middle.
mediated transport
Transport of a substance
across a cell membrane mediated by a
carrier molecule in the membrane.
medulla
(L. marrow). The inner
portion of an organ in contrast to the cortex
or outer portion. Also, hindbrain.
medusa
(Gr. mythology, female
monster with snake-entwined hair). A
jellyfish, or the free-swimming stage in the
life cycle of cnidarians.
Mehlis' gland
Glands of uncertain
function surrounding the ootype of
trematodes and cestodes.
meiofauna
(Gr. meion, smaller, + L. faunus, god of the woods). Small
invertebrates found in the interstices
between sand grains.
meiosis
(Gr. from mieoun, to make
small). The nuclear changes by means of
which the chromosomes are reduced from
the diploid to the haploid number; in
animals, usually occurs in the last two
divisions in the formation of the mature egg
or sperm.
melanin
(Gr. melas, black). Black
or dark-brown pigment found in plant or
animal structures.
melanophore
(Gr. melania, blackness, + pherein, to bear).
Black or brown chromatophore containing
melanin.
memory cells
Population of long-lived B
lymphocytes remaining after initial immune
response that provides for the secondary
response.
meninges
, sing. meninx (Gr. meninx, membrane). Any of three
membranes (arachnoid, dura mater, pia
mater) that envelop the vertebrate brain and
spinal cord. Also, solid connective tissue
sheath enclosing the central nervous system
of some vertebrates.
menopause
(Gr. men, month, + pauein, to cease). In the human female,
that time of life when ovulation ceases;
cessation of the menstrual cycle.
menstruation
(L. menstrua, the menses, from mensis,
month). The discharge of blood and uterine
tissue from the vagina at the end of a
menstrual cycle.
meroblastic
(Gr. meros, part, + blastos, germ). Partial cleavage occurring
in zygotes having a large amount of yolk at
the vegetal pole; cleavage restricted to a
small area on the surface of the egg.
merozoite
(Gr. meros, part, + zoon, animal). A very small trophozoite at
the stage just after cytokinesis has been
completed in multiple fission of a protozoan.
mesenchyme
(Gr. mesos,
middle, + enchyma, infusion). Embryonic
connective tissue; irregular or amebocytic
cells often embedded in gelatinous matrix.
mesentery
(L. mesenterium,
mesentery). Peritoneal fold serving to hold
the viscera in position.
mesocoel
(Gr. mesos, middle, + koilos, hollow). Middle body coelomic
compartment in some deuterostomes,
anterior in lophophorates, corresponds to
hydrocoel in echinoderms.
mesoderm
(Gr. mesos, middle, + derma, skin). The third germ layer,
formed in the gastrula between the
ectoderm and endoderm; gives rise to
connective tissues, muscle, urogenital and
vascular systems, and the peritoneum.
mesoglea
(Gr. mesos, middle, + glia, glue). The layer of jellylike or cement
material between the epidermis and
gastrodermis in cnidarians and ctenophores;
also may refer to jellylike matrix between
epithelial layers in sponges.
mesohyl
(Gr. mesos, middle, + hyle, a wood). Gelatinous matrix
surrounding sponge cells; mesoglea,
mesenchyme.
mesolecithal
(Gr. mesos,
middle, + lekithos, yolk). Pertaining to a
zygote (or ovum) having a moderate amount
of yolk concentrated in the vegetal pole.
mesonephros
(Gr. mesos,
middle, + nephros, kidney). The middle of
three pairs of embryonic renal organs in
vertebrates. Functional kidney of fishes and
amphibians; its collecting duct is a Wolffian
duct. Adj.,
mesonephric
mesosome
(Gr. mesos, middle, +
soma, body). The portion of the body in
lophophorates and some deuterostomes that
contains the mesocoel.
messenger RNA (mRNA)
A form of
ribonucleic acid that carries genetic
information from the gene to the ribosome,
where it determines the order of amino
acids as a polypeptide is formed.
metabolism
(Gr. metabole, change). A group
of processes that includes digestion, production of energy (respiration), and synthesis
of molecules and structures by organisms;
the sum of the constructive (anabolic) and
destructive (catabolic) processes.
metacentric
(Gr. meta,
between, among, after, + kentron, center).
Chromosome with centromere at or near the
middle.
metacercaria
(Gr. meta,
between, among, after, + kerkos, tail, + L. aria, connected with). Fluke juvenile
(cercaria) that has lost its tail and has
become encysted.
metacoel
(Gr. meta, between,
among, after, + koilos, hollow). Posterior
coelomic compartment in some
deuterostomes and lophophorates;
corresponds to somatocoel in echinoderms.
metamere
(Gr. meta, after, + meros, part). A repeated body unit along
the longitudinal axis of an animal; a somite,
or segment.
metamerism
(Gr. meta,
between, among, after, + meros, part).
Condition of being made up of serially
repeated parts (metameres); serial
segmentation.
metamorphosis
(Gr. meta, between, among,
after, + morphe, form, + osis, state of).
Sharp change in form during postembryonic
development, for example, tadpole to frog
or larval insect to adult.
metanephridium
(Gr. meta, between, among, after, + nephros,
kidney). A type of tubular nephridium with
the inner open end draining the coelom and
the outer open end discharging to the
exterior.
metanephros
(Gr. meta,
between, among, after, + nephros, kidney).
Embryonic renal organs of vertebrates
arising behind the mesonephros; the
functional kidney of reptiles, birds, and
mammals. It is drained from a ureter.
metasome
(Gr. meta, after,
behind, + soma, body). The portion of the
body in lophophorates and some
deuterostomes that contains the metacoel.
metazoa
Gr. meta, after, + zoon,
animal). Multicellular animals.
MHC
See major histocompatibility complex.
microevolution
(L. mikros, small, + evolvere, to unfold). A
change in the gene pool of a population
across generations.
microfilament
(Gr. mikros,
small, + L. filum, a thread). A thin, linear
structure in cells; of actin in muscle cells
and others.
microfilariae
(Gr. mikros,
small, + L. filum, a thread). Partially
developed juveniles borne alive by filarial
worms (phylum Nematoda).
microgamete
(Gr. mikros,
small, + gamos, marriage). The smaller of
the two gamete types in a heterogametic
organism, considered the male gamete.
microglial cells
Phagocytic cells in the
central nervous system, part of the
reticuloendothelial system.
micromere
(Gr. mikros, small, + meros, part). The smallest size class of
blastomeres in a cleaving embryo when the
blastomeres differ in size from one another.
micron
(µ) (Gr. neuter of mikros,
small). One one-thousandth of a millimeter;
about 1/25,000 of an inch. Now largely
replaced by micrometer (µm).
microneme
(Gr. mikros, small, + nema, thread). One of the types of
structures composing the apical complex in
the phylum Apicomplexa, slender and
elongate, leading to the anterior and
thought to function in host cell penetration.
micronucleus
A small nucleus found in ciliate
protozoa; controls the reproductive
functions of these organisms.
micropyle
(Gr. mikros, small, + pileos, a cap). The small opening through
which the cells emerge from a gemmule
(phylum Porifera).
microthrix
See microvillus
microtubule
(Gr. mikros, small, + L. tubule,
pipe). A long, tubular cytoskeletal element
with an outside diameter of 20 to 27 µm.
Microtubules influence cell shape and play
important roles during cell division.
microvillus
(Gr. mikros, small, + L. villus,
shaggy hair). Narrow, cylindrical
cytoplasmic projection from epithelial cells;
microvilli form the brush border of several
types of epithelial cells. Also, microvilli with
unusual structure cover the surface of
cestode tegument (also called microthrix [pl. microtriches]).
mictic
(Gr. miktos, mixed or
blended). Pertaining to haploid egg of
rotifers or the females that lay such eggs.
mineralocorticoids
(M. E. minerale, ore, + L. cortex, bark, + oid, suffix denoting likeness of form).
Hormones of the adrenal cortex, especially
aldosterone, that regulate salt balance.
miracidium
(Gr. meirakidion,
youthful person). A minute ciliated larval
stage in the life of flukes.
mitochondrion
(Gr. mitos, a thread, + chondrion, dim. of
chondros, corn, grain). An organelle in the
cell in which aerobic metabolism takes
place.
mitosis
(Gr. mitos, thread, + osis,
state of). Nuclear division in which there is
an equal qualitative and quantitative
division of the chromosomal material
between the two resulting nuclei; ordinary
cell division.
molecule
A configuration of atomic nuclei and
electrons bound together by chemical
bonds.
monocyte
(Gr. monos, single, + kytos, hollow vessel). A type of leukocyte
that becomes a phagocytic cell
(macrophage) after moving into tissues.
monoecious
(Gr. monos, single, + oikos, house). Having both male and
female gonads in the same organism;
hermaphroditic.
monogamy
adj. monogamous (Gr. monos, single, + gamos, marriage).
The condition of having a single mate at
any one time.
monohybrid
(Gr. monos, single, + L. hybrida, mongrel). A hybrid offspring of
parents different in one specified character.
monomer
(Gr. monos, single, + meros, part). A molecule of simple
structure, but capable of linking with others
to form polymers.
monophyly
(Gr. monos, single, + phyle, tribe). The condition that a taxon
or other group of organisms contains the
most recent common ancestor of the group
and all of its descendants; contrasts with polyphyly and paraphyly
monosaccharide
(Gr. monos, one, + sakcharon, sugar, from
Sanskrit sarkara, gravel, sugar). A simple
sugar that cannot be decomposed into
smaller sugar molecules; the most common
are pentoses (such as ribose) and hexoses
(such as glucose).
monozoic
(Gr. monos, single, + zoon, animal). Tapeworms with a single
proglottid, do not undergo strobilation to
form chain of proglottids.
morphogenesis
(Gr. morphe, form, + genesis, origin).
Development of the architectural features of
organisms; formation and differentiation of
tissues and organs.
morphology
(Gr. morphe, form, + L. logia,
study, from Gr. logos, work). The science of
structure. Includes cytology, the study of
cell structure; histology, the study of tissue
structure; and anatomy, the study of gross
structure.
morula
(L. morum, mulberry, + ula, dim.). Solid ball of cells in early
stage of embryonic development.
mosaic cleavage
Embryonic development
characterized by independent differentiation
of each part of the embryo; determinate
cleavage.
mucin
(L. mucus, nasal mucus). Any
of a group of glycoproteins secreted by
certain cells, especially those of salivary
glands.
mucus
(L. mucus, nasal mucus).
Viscid, slippery secretion rich in mucins
produced by secretory cells such as
those in mucous membranes. Adj.,
mucous
Müller's larva
Free-swimming ciliated larva that
resembles a modified ctenophore,
characteristic of certain marine polyclad
turbellarians.
multiple fission
A mode of asexual
reproduction in some protistans in which
the nuclei divide more than once before
cytokinesis occurs.
mutation
(L. mutare, to change).
A stable and abrupt change of a gene; the
heritable modification of a characteristic.
mutualism
(L. mutuus,
lent, borrowed, reciprocal). A type of
interaction in which two different species
derive benefit from their association and in
which the association is necessary to both;
often symbiotic.
myelin
(Gr. myelos, marrow). A
fatty material forming the medullary sheath
of nerve fibers.
myocyte
(Gr. mys, muscle,+ kytos,
hollow vessel). Contractile cell (pinacocyte)
in sponges.
myofibril
(Gr. mys, muscle, + L. dim. of fibra, fiber). A contractile filament within
muscle or muscle fiber.
myogenic
(Gr. mys, muscle, + N.L., genic, giving rise to). Originating in
muscle, such as heartbeat arising in
vertebrate cardiac muscle because of
inherent rhythmical properties of muscle
rather than because of neural stimuli.
myomere
(Gr. mys, muscle, + meros, part). A muscle segment of
successive segmental trunk musculature.
myosin
(Gr. mys, muscle, + in,
suffix, belonging to). A large protein of
contractile tissue that forms the thick
myofilaments of striated muscle. During
contraction it combines with actin to form
actomyosin.
myotome
(Gr. mys, muscle, + tomos, cutting). That part of a somite
destined to form muscles; the muscle group
innervated by a single spinal nerve.
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