Encyclopedia of Biology

to aslight excess of positive ions on one side and of
negative ions on the other; potential inside a membrane
minus the potential outside. A typical membrane poten-
tial is –60 mV, where the inside is negative relative to
the surrounding fluid, and resting membrane potentials
are typically found between –40 and –100 mV.

memory cell Cells (lymphocytes) that have been
exposed to specific antigens and remain in the body
after an immune response to attack those same antigens
if reexposed to them in the future. Memory cells are
subsets of T and B cells.

Mendel, Gregor JohannMendel (1822–84) was an
Austrian botanist and monk who was the first to lay
the groundwork for the foundation of the science of
genetics, using his now famous experiments with
breeding peas at his monastery. His groundbreaking
research paper “Experiments in Plant Hybridization,”
was read at a meeting on February 8, 1865. He
concluded that genes were not blends of parental
traits, but instead were separate physical entities
passed individually in specific proportions from one
generation to the next.

Menkes’ disease A sex-linked inherited disorder,
causing defective gastrointestinal absorption of copper
and resulting in copper deficiency early in infancy.

menstrual cycle The cyclical growth and destruction

of the female endometrium each month; a stage of the

female reproductive cycle. As an egg matures and is

released monthly, hormones such as estrogen stimulate

thethickening of the endometrium. Progesterone stops

the growth of the endometrium and prepares the body

for pregnancy. If it does not occur, the endometrium

becomes a bloody discharge through the cervix into the

vagina, thus ending the menstrual period.

See alsoESTROUS CYCLE.

meristem A group of plant cells that can divide

indefinitely and can provide new cells for the plant as

long as it lives.

meroblastic cleavage(incomplete cleavage) Atype

of cleavage where only part of the fertilized egg, the

blastodisc, goes through division, usually leaving a

large concentration of yolk in the egg; observed in

avian development.

mesentery Amembrane fold (peritoneum) suspend-

ing, attaching, and anchoring various organs to the

body cavity, such as the small intestine and spleen;

provides blood, lymphatic vessels, and nerve supply to

and from the organs. The term mesentery is used

generically describing peritoneal extensions not only

from the intestine but from all abdominal and pelvic

organs.

mesoderm The middle (mesos) of three germ layers

(endo- and ectoderms are the other two) of the early

embryo during gastrulation, early mammalian embry-

onic development, that gives rise to blood, cartilage,

circulatory system, connective tissue, gonads, kidneys,

and muscle. Three regions of the mesoderm are respon-

sible for specific parts: the epimere or dorsal mesoderm

forms somites, discrete clumps of mesoderm, which

forms the connective tissue layer of the skin, most of

the bony skeleton, and most of the striated musculature

(each somite is further split into dermotome, myotome,

and sclerotome segments); the mesomere or intermedi-

ate mesoderm, which differentiate into the kidney and

216 memory cell

xylem phloem vein cuticle

adaxial

epidermis

abaxial

epidermis

palisade layer

spongy layer

cuticle

stomatal pore guard cell

air space

mesophyll

The ground tissue of a leaf, located between the upper and lower
epidermis, specialized for photosynthesis is called the mesophyll.