Mosaic development
A pattern of development, such as that of a mollusk, in which the early blastomeres each
give rise to a specific part of the embryo. In some animals, the fate of the blastomeres is
established in the zygote.
Mosaic evolution
The evolution of different features of an organism at different rates.
Motor neuron
A nerve cell that transmits signals from the brain or spinal cord to muscles or glands.
Motor unit
A single motor neuron and all the muscle fibers it controls.
Morula
The spherical embryonic mass of blastomeres formed before the blastula and resulting from
cleavage of the fertilized ovum
Moulting
To shed periodically part or all of a coat or an outer covering, such as feathers, cuticle, or
skin, which is then replaced by a new growth.
MPF (M-phase promoting factor)
A protein complex required for a cell to progress from late interphase to mitosis; the active
form consists of cyclin and cdc2, a protein kinase.
Muscle fiber
Muscle cell; a long, cylindrical, multinucleated cell containing numerous myofibrils, which is
capable of contraction when stimulated.
M Phase
The entire process of cell division including division of the nucleus and the cytoplasm.
mRNA
RNA, synthesized from a DNA template during transcription, that mediates the transfer of
genetic information from the cell nucleus to ribosomes in the cytoplasm, where it serves as
a template for protein synthesis. Also called messenger RNA.
Mucilage
A gummy substance obtained from certain plants.
Mullerian mimicry
A mutual mimicry by two unpalatable species.
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