Glossary 349
composed almost entirely of collagen. It also forms the basic structure of
bone. See also Apatite.
Crystal lattice: The precise, orderly arrangement of atoms in a crystal, forming
a netlike structure.
Dedifferentiation: The process in which a mature, specialized cell returns to
its original, embryonic, unspecialized state. During dedifferentiation the
genes that code for all other cell types are made available for use by de-
repressing them. See also DIFFERENTATION, Gene, Redifferentiation.
Dendrite: The prolongation of a nerve cell that carries a message, or stimulus,
toward the cell body. For example, sensory nerve cell bodies receive stimuli
from receptors in the skin via their dendrites. See also Axon, Neuron.
Differentiation: The process in which a cell matures from a simple embryonic
type to a mature, specialized type in the adult. Differentiation involves
restricting, or repressing, all genes for other cell types. See also De-
differentiation, Gene, Redifferentiation.
DNA: The molecule in cells that contains genetic information.
Ectoderm: One of three primary tissues in the embryo, formed as differentia-
tion (cell specialization) is just beginning. The ectoderm gives rise to the
skin and nervous system. See also Endoderm, Mesoderm.
Electrode: A device, usually metal, that connects electronic equipment to a
living organism for the purpose of measuring electrical currents or vol-
tages in the organism, or delivering a measured electrical stimulus to the
organism.
Electrolyte: Any chemical compound that, when dissolved in water, separates
into charged atoms that permit the passage of electrical current through
the solution.
Embryogenesis: The growth of a new individual from a fertilized egg to the
moment of hatching or birth.
Endoderm: One of three primary tissues in the embryo, formed as differentia-
tion (cell specialization) is beginning. It forms the digestive organs. See
also Ectoderm, Mesoderm.
Epidermis: The outer layer of skin, having no blood vessels.
Epigenesis: The development of a complex organism from a simple, un-
differentiated unit, such as the egg cell. It is the opposite of preformation,
in which a complex organism was thought to develop from a smaller, but
similarly complex, antecedent, such as the homunculus that some early
biologists thought resided in the sperm or egg cell.
Epithelium: A general term for skin and for the lining of the digestive tract.
Exudate: Liquid, sometimes containing cells, that diffuses out from a wound or
surface structure of a living organism. Examples are a wound exudate and
the slime exudate from the skin of fish.